However, Welbeck helped swing the tie back in the Gunners' favour on 62 minutes, before Angel Di Maria's red card undermined United on the comeback trail.
Below, relive the drama at the Theatre of Dreams with our live commentary.
Good evening ladies and gentlemen and welcome to our live text coverage of Manchester United's FA Cup quarter-final clash with Arsenal at Old Trafford. Whether it all goes a bit Jeremy Kyle out on the field, or ends with Sir Alex Ferguson wearing a pizza in the tunnel, this is a fixture that rarely fails to entertain - and we've got the added spice of a semi-final place up for grabs tonight! Let's go.
Team news is coming up shortly but first let's take a ramble down memory lane and reflect on the classic ties that his competition has thrown up, shall we? Okay, and it is nigh-on impossible to think about this fixture in the FA Cup without thinking of Ryan Giggs - and, more specifically, his ridiculous, Austin Powers-like chest hair.
Yep, the Welshman wrote his name into FA Cup folklore by turning Arsenal's defence inside out en route to firing United to an extra-time win over the Gunners in the semi-final back in 1999. Warning: if you are a Gooner, or don't particularly want to see an abundance of hair that makes Fellaini's barnet look like a crew cut, then you probably shouldn't watch the following footage.
The North Londoners will have fond memories of this fixture too, of course, having beaten United on penalties to clinch the title back in 2005. It was perhaps one of the most one-sided FA Cup finals you're likely to see as the Red Devils dominated for 120 minutes, before the Gunners triumphed 5-4 on penalties.
In fact, did you know that Arsenal won the first-ever FA Cup meeting between these sides? Yep, a 3-2 win at Old Trafford way back in 1906, when Arsene Wenger was just two years old. That last part may not be true. Anywho, that fixture occurred on March 10, meaning that it's nearly 109 years ago to the day!
By virtue of the fact that they have since won the FA Cup 11 times apiece - more than any other club - it is fair to say that both sides have a certain affinity with this competition, so let's just hope that this one throws up an encounter befitting of the clubs' history against one another. A bit of needle wouldn't go amiss either though, would it?
Right, that's enough of that. We've got team news on the way so stay tuned to see how both sides line up, but first let's whet the appetite, shall we? Why not.
MANCHESTER UNITED XI: De Gea, Valencia, Smalling, Rojo, Shaw, Blind, Herrera, Di Maria, Fellaini, Young, Rooney
ARSENAL XI: Szczesny, Bellerin, Mertesacker, Koscielny, Monreal, Coquelin, Cazorla, Oxlade-Chamberlain, Ozil, Alexis, Welbeck
MANCHESTER UNITED SUBS: Valdes, Rafael, Jones, Carrick, Januzaj, Mata, Falcao
ARSENAL SUBS: Martinez, Gibbs, Chambers, Ramsey, Giroud, Walcott, Akpom
Let's be polite and start with the visitors and I can tell you that Arsene Wenger makes five changes from the 2-1 win over QPR last time out.
Wojciech Szczesny displaces David Ospina between the sticks, while Laurent Koscielny returns to replace the injured Gabriel and partner Per Mertesacker at centre-back. Kieran Gibbs drops out for Nacho Monreal at left-back, while Alex Oxlade-Chamberlain comes into the midfield for Tomas Rosicky. Finally, striker Olivier Giroud is replaced by Danny Welbeck as the former United man leads the line in his first game at Old Trafford since leaving for £16m in September. Big night for him.
On to Louis van Gaal's side now and the Dutchman makes just the one change to his team from their last-gasp 1-0 win at Newcastle United. Jonny Evans serves the first of a six-game ban for spitting at Papiss Cisse last Wednesday as the Northern Irishman is replaced by Luke Shaw at left-back.
No Robin Van Persie tonight either of course, with the former Arsenal hitman sidelined with an ankle injury, as Wayne Rooney spearheads the attack, with Marouane Fellaini supporting in behind it seems.
Now let's have a quick look at how both teams have fared in the Premier League thus far. They've both done alright it has to be said - and there is virtually nothing to choose between them!
We'll start with the Gunners and, at this moment in time, Arsenal sit third in the table, one point clear of fourth-placed United. Wenger's side are actually one of the in-form teams at the moment; boasting 10 victories from 12 league and cup outings in 2015. They come into this on the back of a 2-1 win away to QPR and, simply put, Van Gaal's boys will need to play well to win.
And what about United? One point behind Arsenal and two clear of Liverpool in the race for Champions League qualification, the Red Devils look on course for a top-four finish, which is essential. Having embarked on the second most expensive spending spree in football history last summer, failure to qualify for Europe's elite competition would, in truth, represent failure on Van Gaal's part.
They take on the Londoners after narrowly beating Newcastle 1-0 last Wednesday, with Ashley Young netting a last-gasp winner to keep them in the hunt for Champions League qualification. But who's the one to look out for tonight from an Arsenal perspective? It has to be Wayne Rooney and here's why:
With 13 goals, Rooney has found the net more times against Arsenal than any other team with the exception of Aston Villa (14). Furthermore, the 29-year-old has struck seven times in United's last eight FA Cup outings and Arsenal should be worried about the England captain tonight.
With Theo Walcott, Alexis Sanchez and Mesut Ozil, it's not like Wenger will be fielding a team full of nobodies, is it? But who's the most dangerous? Arguably Alexis, who has 18 goals in all competitions and has looked a real livewire regularly throughout his maiden season. He's a class act.
We'll also be keeping an eye on Giroud because a) he's a rather beautiful man and it's hard not to look at him, and b) he has scored seven and set up four in his last 11 FA Cup appearances. #form #danger #needlesshashtags. He may be only be a substitute tonight, but here's what happened the last time he came off the bench against United.
Prediction! By virtue of the fact that Arsenal have not won at Old Trafford since 2006, it takes a brave man to back them here. However, I am that brave man this evening and I'm on a 2-1 away win. It's Cheltenham eve, so I am reluctant to put my money where my mouth is, but I think the Gunners have the know-how to negotiate tricky ties like this, having won the competition last season, and I believe they'll sneak through to the semis. 10 minutes to go!
Martin Keown is pitchside this evening, which means it gives us another reason to post this. If you pause it at the right moment, you can see D̶r̶ ̶B̶r̶u̶c̶e̶ ̶B̶a̶n̶n̶e̶r̶ Martin try to cage the beast that Ruud van Nistelrooy still has nightmares about.
Right, both sets of players enter the field. Per Mertesacker leads the Gunners, while Rooney leads the Red Devils. Two minutes!
KICKOFF: Aaaaaaaaaaaaaaaaand we're off at Old Trafford. Arsenal, in their blue kit, get us underway. Michael Oliver is our referee of the night, but let's hope that we're left talking about the football rather than a refereeing error.
Both sides have seen a bit of the ball in these opening stages. Nacho Monreal did some good things down Arsenal's left-hand flank there but ultimately conceded a throw-in.
SHOT! We highlighted him before the break, and Alexis Sanchez has had the game's first shot. He dropped the shoulder to deceive Antonio Valencia before firing a shot at goal, but Chris Smalling's block took the sting out of it somewhat and David De Gea gathered.
YELLOW CARD: Hector Bellerin becomes the first player booked after just four minutes for chopping down Ashley Young. That puts him on a tightrope for the rest of the game - especially against a winger like Young.
Danny Welbeck heads away the resulting free kick, much to Arsenal's relief because they only had 10 men on the field as Francis Coquelin received treatment for a bloody nose.
United definitely on top over the last few minutes as Arsenal really struggle to keep possession. Both sides have contested 13 FA Cup ties, with Arsenal winning six and United winning seven. There's a stat for you because there's very little to report.
Better from Arsenal as Welbeck wins his side a corner after latching on to Alexis's pass. Cazorla will whip her in, but it comes to nothing as Rojo heads away. Woeful delivery it has to be said.
This is excellent from David De Gea. Daley Blind does him no favours at all with a dodgy back pass, which Welbeck does his all to pressure him, but the Spanish keeper stays calm manages to clear and find his teammate Valencia with a precise pass in the process. Brilliant.
CHANCE! Big chance for United as Fellaini latches on to Chris Smalling's through ball but, instead of shooting first time with his left, the big Belgian turns in a bid to get it on to his right and then leaves the ball behind. He needed to hit that first time. Poor.
Better from Arsenal in recent minutes. 16 minutes in and Alexis is definitely the go-to-guy for them so far. They've looked to find the tricky Chilean quite often so far. He makes them tick but still 0-0. It's never finishing 0-0, though; you can tell.
CHANCE! Another very decent chance for United, and Di Maria, who's been quiet so far, is the architect as he plays a wonderful over-the-top ball for Fellaini's, whose chest-and-touch falls to Young as the winger blazes over from an angle. Should've done better.
Really poor from Oxlade-Chamberlain in arguably Arsenal's best attack so far. He's released down the right-hand side but his delivery into the box is shocking and the chance goes begging. Disappointing.
GOAL!!!!!!!!!!! MANCHESTER UNITED 0-1 ARSENAL (Nacho Monreal)
We've just slammed Oxlade-Chamberlain but he does so brilliantly to dribble around a few challenges before playing in Monreal, as the Spaniard casually slots past his compatriot at the near post to give Arsenal a 1-0 lead!
United won't be too disappointed because a) it's still early days and b) they've looked dangerous in the final third, especially down Arsenal's right hand-side. It's definitely not a reason to throw the baby out with the bath water.
GOAL!!!!!!!!!!! MANCHESTER UNITED 1-1 ARSENAL (Wayne Rooney)
What a response from United and they're back on level terms almost immediately. It's a brilliant delivery from Angel di Maria as Rooney, who sneaked in between Koscielny and Mertesacker, plants a header beyond the helpless Szczesny. We did say there would be fireworks didn't we?
PENALTY APPEAL: Danny Welbeck incurs the wrath of his former fans by going to ground inside the box under a challenge from Marcos Rojo. His shirt was certainly being tugged but I don't know; it's one of them grey area appeals.
Arsenal will look to get to half time at 1-1 because they have looked a little shaky at times back there in defence. They need half time far more than United at the moment.
OVER THE BAR: Chris Smalling does well to connect with Ashley Young's corner but his header flies over the crossbar.
YELLOW CARD: We've seen the second card of the night and it's Ander Herrera for his foul on Alexis. Right decision again from Michael Oliver, who has had quite a good game so far.
The 9,000 travelling fans in good voice as they urge their team on here. It's an excellent game so far it has to be said.
SHOT! Excellent initial play from Rooney, who dinks a wonderful ball over to Di Maria, who can't get a shot away but Rooney picks up the pieces with an attempted chip that Szczesny comfortably takes.
SAVE! Good save by Szczesny to keep out Di Maria's right-footed effort. The outcome might've been different had it come on the Argentine's left, but Arsenal survive nonetheless. The Gunners have a free kick about 35 yards from goal now and I think Cazorla is going for goal...
Yep, Cazorla's effort is on target but that's the easiest save De Gea will make all night. It was too far out to really test a goalkeeper of De Gea's quality.
TWO MINUTES ADDED ON:
HALF TIME: MANCHESTER UNITED 1-1 ARSENAL
That's all she wrote in the first half my friends and it's been a really good game so far. Both sides could be better, but yeah, very decent first half. Nacho Monreal's opener was cancelled out by Wayne Rooney a matter of minutes after breaking the deadlock....
From an Arsenal perspective, we need to see Theo Walcott and/or Olivier Giroud at some point in this second half because those two are the only real goal threats they have on the bench. With United, they've got the likes of Radamel Falcao and Juan Mata on the bench so both sides have plenty of quality in reserve if they need it.
There's two United subs to tell you about as Phil Jones and Michael Carrick replace Luke Shaw and Ander Herrera, which means we won't see Mata AND Falcao tonight; just one of them. No changes for the Gunners.
RESTART: Aaaaaaaaaaaaaaaaaand we're back. United, playing from right to left in this half, get us back underway at Old Trafford. It's a massive 45 minutes in both these teams' seasons really. Let's go.
Looks like Phil Jones has gone to centre-back, allowing Rojo to switch to left-back, where he played at Newcastle last week. It'll be interesting to see if Oxlade-Chamberlain has more or less joy against Rojo than he did against Shaw, whom he did get the better of at times in that first half.
Some worrying signs for Arsenal as Oxlade-Chamberlain pulls up after grabbing his calf and we're probably going to see Theo Walcott now. It's a shame for Oxlade-Chamberlain, who has been good tonight.
ARSENAL SUB: Walcott was stripped and ready to come on, but it's Aaron Ramsey believe it or not. Presumably Ozil will go out wide and Ramsey in the middle. If Arsenal lose, and that sub proves decisive, then I wouldn't want to be Arsene Wenger tonight. Twitter will go into overload.
CHANCE! Arguably Arsenal's best chance of the game (apart from the goal) as Cazorla feeds the unmarked Alexis on the Gunners' left wing, but the Chilean's shot is deflected and De Gea comfortably saves. Has anybody seen Antonio Valencia? He's been caught with his pants down more times than....I'm not going to finish that sentence. Suffice to say, United's right-back hasn't always been at right-back tonight.
Olivier Giroud is out warming up, jogging along the touchline as both males and females stop and admire the Frenchman. Welbeck hasn't offered much at all on his return to Old Trafford, so we should be seeing Giroud soon.
BOOKING: Fellaini finally goes into the book for a foul on Ramsey. We've seen three bookings tonight and they've all been right - two for United, one for Arsenal. Fair play to Michael Oliver.
BOOKING: 3-1 to United in the yellow card stakes now as Ashley Young follows Fellaini, Herrera and Bellerin into the book.
CHANCE! It's 3-1 in the yellow card stakes, and it should be 2-1 in the, eh, goal stakes? Angel di Maria latches on to Rooney's clipped pass and drills just wide.
GOAL!!!!!!!!!!!!!!!!!!!!!! MANCHESTER UNITED 1-2 ARSENAL (Welbeck)
Moments after Di Maria misses a good chance, Arsenal take full advantage as Welbeck capitalises on a loose back pass from Valencia, before knocking it around De Gea and calmy finishing into an empty net. Good goal and Arsenal in the driving seat once more. Valencia has had a shocker tonight.
BOOKING: Bellerin, who has already been booked, gets away with one there after fouling Ashley Young, who attempts to convince Michael Oliver to send off the young Spaniard but he's having none of it. Marcos Rojo is the player who was booked in the end.
CHANCE! It could, and should, be 2-2 as Smalling taps over from four yards out! Big miss.
ARSENAL SUB: Wenger makes his second sub as Calum Chambers replaces Bellerin. Probably wise, but time will tell because Chambers hasn't looked the best of right-backs this season.
We've reached the 70-minute mark and what a big last 20 minutes it is for both sides. With Liverpool the only formidable team left in the FA Cup - and even they face a quarter-final replay at Blackburn - then victory for Arsenal would see them take a huge step toward retaining their title in May.
Oh dear Valencia really is having a shocker. He fouls Alexis just outside the box and the Chilean will look to whip this into the top corner...
SAVE! He goes for goal, but De Dea does very well to catch and hold the ball because Welbeck was ready to capitalise on any slip.
SUBS: Marcos Rojo makes way for Adnan Januzaj, while Olivier Giroud replaces Danny Welbeck. Boos ring around Old Trafford after the Arsenal striker celebrated what could yet prove the winning goal against his former club.
SAVE!!!! We know how good David De Gea is, but this is unbelievable. Sanchez floats a wonderful cross into the box and Cazorla's volley is heading into the far post, but De Gea, out of nowhere, leaps across and keeps it out. Incredible.
RED CARD: Ridiculous from Di Maria, who is sent off for two yellow cards in the space of about 30 seconds. Firstly, he's booked for diving, before grabbing Michael Oliver's shirt in disgust over the decision and he's gone! So petulant and Arsenal are massive favourites now.
It would be very Arsenal to throw this away, though, so this one is far from over I'm afraid. And United have a free kick right on the edge of the box now after Monreal's handball!
SHOT! Rooney takes on the free kick, but it's too close to get it up and over the wall as the United skipper curls over the bar.
The reality for United is that, even if they do score, they've only got 10 men and three defenders on the field, so you couldn't back against Arsenal scoring again, could you? Seven minutes to go.
© Getty Images
Here's the moment Welbeck came back to haunt Manchester United, but will it prove to be the winning goal? Only time will tell. Five minutes of normal time on the clock.
Here's the moment Welbeck came back to haunt Manchester United, but will it prove to be the winning goal? Only time will tell. Five minutes of normal time on the clock.
BOOKING: Michael Oliver is having a blinder as Adnan Januzaj goes into the book for simulation in the box. Fair play to him because it takes a bit of bottle to book two United players for diving at Old Trafford.
BOOKING: Aaron Ramsey goes into the book now for cynically fouling Ashley Young. Another good decision from Oliver.
FIVE MINUTES ADDED ON: Plenty of time for United to nick an equaliser as the fourth official shows up five minutes of additional time. Can they do it? Let's see.
United fans still in good voice as they look to Spur on their team for the last three minutes but it's proving futile so far.
CHANCE! Alexis Sanchez fires agonisingly wide of the post as Arsenal miss the chance to kill this one off once and for all. It might've been a save actually.
FULL TIME: MANCHESTER UNITED 1-2 ARSENAL
That's it, my friends. Danny Welbeck is the hero as Arsenal beat 10-man Manchester United 2-1 to advance into the semi-finals of the FA Cup.
Angel Di Maria was sent off for two yellow cards in the space of about 30 seconds and that seemed to take the energy out of United on the comeback trail.
We will have the results of the FA Cup semi-final draw coming up on Sports Mole very soon, but for now we must part ways. Thanks for joining us and we'll see you next time.