Both sides are winless in two Premier League matches, but it is the Blues who sit in a healthier position as they occupy the final Champions League qualification spot, three points better off than their opponents.
United's 11-match unbeaten run to kick off the Ole Gunnar Solskjaer era, 10 of those games ending in victory, now seems a long time ago.
The Red Devils have badly tailed off and they are currently on their worst run of form in 57 years, having lost seven of their last nine matches in all competitions, including back-to-back losses in the Premier League by a 6-0 aggregate scoreline.
With their Champions League campaign being ended by Barcelona earlier this month, United were supposed to have been given an advantage over the sides in and around them, each of whom are still competing in Europe.
It has not quite turned out that way, though, and now this weekend's showdown with Chelsea is seen as a season-saving match. Win it and the Red Devils will be right back in the mix for a top-four finish; lose and Solskjaer's problems will get a whole lot worse.
The good news for Solskjaer is that his side are still in the hunt for fourth place, despite collecting just six points from the last 18 on offer.
That is because, between them, Tottenham Hotspur, Arsenal, Chelsea and United have earned just one draw and suffered five defeats in their last six matches, none of them able to drag themselves over the line.
After Sunday's difficult test against Chelsea, United have two winnable fixtures to conclude their campaign against Huddersfield Town (A) and Cardiff City (H).
Solskjaer will know that he can still achieve the target he was set by his bosses when being awarded a permanent contract, then, by overtaking Arsenal - two points better off ahead of their trip to Leicester City - and Chelsea in the remaining fortnight.
After seeing their side ship nine goals in their last three matches, however, and without winning back-to-back matches in nearly two months, the home supporters in attendance at Old Trafford have every right to feel anxious.
Recent form in Premier League: LWLWLL
Recent form (all competitions): LLWLLL
Chelsea headed into Monday night's home match with lowly Burnley knowing that victory would take them four points clear of closest challengers Arsenal.
The 2-2 draw at Stamford Bridge neatly summed up what has been a strange battle for fourth place, though, as the Blues failed to take their big chance, leaving them in need of a positive result from this trip to Old Trafford.
Maurizio Sarri will not need telling that his side have lost their last six Premier League away games against fellow big-six sides, including all four this season by an aggregate score of 13-1. Indeed, they are winless in eight such games since beating Tottenham 2-1 at Wembley Stadium in August 2017.
The Blues have been far too inconsistent on their travels for a while now, losing five of their last seven matches away from Stamford Bridge, with the only wins in that run coming away to bottom-three sides Fulham and Cardiff.
With the pressure building on Sarri, and fresh reports surfacing that he will be on his way in the summer, this could well be a make-or-break game for the Italian.
Chelsea have not gone three Premier League games without a win since September 2016, which they followed up with a 13-match winning streak, and another loss on Sunday could well put the wheels in motion for a replacement to be brought in.
The West London outfit do still have the small matter of a Europa League semi-final against Eintracht Frankfurt to come, though, meaning that unlike United, they can still reach the Champions League via other means.
If they are to achieve their aim of bringing Champions League football back to Stamford Bridge, they are going to have to do it the hard way. Fourth place may be Chelsea's to lose, but they have trips to Man United and Leicester City to come, as well as hosting FA Cup finalists Watford.
The top-four picture could look completely different come Sunday evening. If Sarri is to hang on to his job, his side cannot afford to taste defeat in Manchester this weekend.
Recent form in Premier League: LWWWLD
Recent form (all competitions): WWWLWD
Chelsea lost Callum Hudson-Odoi to an Achilles injury earlier this week, and they could be without the teenage forward for the rest of 2019.
Willian sat out the Burnley match with a thigh problem but is back in contention, though N'Golo Kante left the field with a back complaint and is in a race against time to be fit for this huge showdown with United.
Elsewhere, Sarri is confident of having Antonio Rudiger back available for selection after being without the defender for two weeks, and squad players Marcos Alonso and Ethan Ampadu are also back in training.
As for the hosts, Ander Herrera's prolonged absence with a hamstring injury may be about to come to an end as he has spent the last few days training.
Phil Jones was withdrawn during the shocking 4-0 loss to Everton and remains a doubt, and there remain question marks over the fitness of Antonio Valencia and Eric Bailly, who have not been used since December and February respectively.
Solskjaer made some surprise selection decisions for the Manchester derby, and it remains to be seen whether he will revert back to his tried-and-trusted formula this weekend.
Manchester United possible starting lineup:
De Gea; Young, Smalling, Lindelof, Shaw; Herrera, Matic, Pogba; Lingard; Rashford, Lukaku
Chelsea possible starting lineup:
Kepa; Azpilicueta, Rudiger, Luiz, Emerson; Kante, Jorginho, Kovacic; Pedro, Higuain, Hazard
Head To Head
Man United have won their last two Premier League home games against Chelsea - they have not beaten the Blues in three top-flight games in a row at Old Trafford since January 1957, when Matt Busby was in charge.
The Red Devils have also won three of their last six meetings with Chelsea in all competitions (D1 L2), including February's 2-0 win in the FA Cup last 16 at Stamford Bridge.
However, the Blues have won more top-flight games against United than any other side in the competition (18).
We say: Manchester United 1-1 Chelsea
Chelsea's six Premier League wins at Old Trafford have come under six different bosses, with five of those victories being the manager's first such game at the ground with the Blues. That is a good omen for Sarri, and his side make the trip to the Theatre of Dreams at the perfect time, coming up against a United side devoid of any confidence. However, the visitors are hardly at their best, so we are backing a draw that will take the top-four battle right down to the wire.