Manchester United have been knocked out of the EFL Cup in the third round for the first time since 2007 courtesy of a penalty shootout defeat at the hands of Championship side Derby County this evening.
The hosts took the lead through Juan Mata after only three minutes, but a goal-of-the-season contender from Harry Wilson levelled things up in the second half before United were reduced to 10 men when goalkeeper Sergio Romero handled the ball outside his area.
Jack Marriott seemed to have set the Rams on course for victory with an 85th-minute header, but United took full advantage of six minutes of injury time as Marouane Fellaini nodded home an equaliser from close range in the fifth added minute to rescue a penalty shootout.
However, it only proved to be a temporary reprieve as Phil Jones saw his spot kick saved after 15 successful penalties in a row, securing Derby's first win at Old Trafford since 2001.
Mourinho made nine changes to his side from the weekend draw against Wolverhampton Wanderers, with Paul Pogba, Alexis Sanchez and Luke Shaw among those to drop out as Romelu Lukaku and Jesse Lingard were the only players retained.
Both had a role in the opening goal, but United could have taken the lead even before that third-minute strike as they made a flying start to the match.
Anthony Martial - who tormented the Derby defence for much of the first half - swung a dangerous cross into the box which Lukaku met with a glancing header that Scott Carson needed to save in the Derby goal.
The hosts did have the ball in the back of the net just seconds later as Martial's pass was dummied by Lukaku into the path of Lingard, who in turn slipped Mata on the right channel. Mata took the shot on first time, curling the ball past Carson and into the bottom corner for his first goal since December.
It was a perfect start for United, but Derby almost produced an equally good response moments later when Mason Mount's incisive low pass into the box made it all the way through to Mason Bennett, who was thwarted by a fine save from Romero as an unmarked Wilson went ignored in the middle.
The bulk of the early chances fell to United, though, and Mata could have doubled his personal tally in the ninth minute when he beat the offside trap and latched on to Ashley Young's pass forward, only for Carson to deny the Spaniard on the turn.
The resulting corner almost snuck straight in as only the near post spared Carson's blushes, and another corner seconds later was met by Nemanja Matic, who planted his header off target.
Another golden opportunity went begging for the hosts after 13 minutes when Richard Keogh's heavy touch allowed Lukaku to race clean through on goal, but Fikayo Tomori raced across to help his centre-back partner and did just enough to force Lukaku to fire his effort wide.
The frenetic pace of the match eventually relented, but there were still sights of goal at both ends and Craig Bryson sent one 20-yard strike bobbling a couple of yards wide.
Martial continued to look the most likely to make something happen for United at the other end, and he reached the byline midway through the half before standing the ball up for Lukaku, who could not keep his header down from point-blank range having seen Carson taken out of the game by the cross.
Eric Bailly then flicked a corner wide of the near post, but Derby looked more than a match for their illustrious hosts for the remainder of half, seeing plenty of the ball inside United territory without producing the quality to make the most of that.
The Championship outfit also made a positive start to the second half and almost levelled things up within eight minutes of the restart when Wilson drilled a low effort across goal which Romero got just enough on to turn past the post.
The equaliser did come just before the hour mark, though, and it arrived in some style as Wilson - on loan from Liverpool - fired a swerving and dipping free kick into the top corner from all of 35 yards, leaving a helpless Romero rooted to the spot.
Suddenly Derby's tails were up and Bradley Johnson tried his own luck from range with an effort which sailed over the bar before Martial fired into the side-netting at the other end moments later.
Things soon went from bad to worse for United, though, as Romero was given his marching orders for handling the ball outside his area after Wilson had got to a pass before the keeper and looked to poke it beyond him.
Grant would go on to have a noteworthy impact on the match, but not before Lukaku had thumped a shot against the post and Martial had curled an effort narrowly wide seconds later and United fired a warning that they could still carry a threat even with 10 men.
Mourinho had used all three subs before Lampard turned to his bench, and the Derby manager's first change almost had an immediate impact when Florian Jozefzoon powered a downward header towards the bottom corner from Omar Bogle's cross, only for Grant to produce an even better save.
Martial then snatched at an opening after being teed up by Fellaini eight minutes from time, and that looked as though it would be costly when Derby took the lead shortly afterwards through Marriott's first goal for the club.
The striker - on as a sub for David Nugent just six minutes earlier - had seen his initial shot blocked before falling to Mount, whose low first-team effort from the edge of the box was well saved by Grant. However, Marriott continued to follow in and nodded home the rebound despite Grant's best efforts, with the goal being confirmed after a brief check with VAR.
Martial lashed another effort over the top in the final minute of the 90 before six minutes of additional time were indicated by the fourth official - a bonus United took full advantage of.
Again it was a sub who made the difference as Fellaini latched on to home debutant Diogo Dalot's cross and nodded home from close range, rescuing a penalty shootout for the hosts at the death.
It proved to be a shootout of the highest standard as the first 15 players all scored - most doing so in style too - until Jones saw his spot kick saved by Carson to seal an 8-7 win from the visitors.
The result means that United have now failed to win in three successive home matches for the first time under Mourinho, while Derby progress from a cup tie against United for the first time in their history.
MAN UTD (4-3-3): Romero; Dalot, Bailly, Jones, Young; Mata (Grant 70'), Matic, Herrera (Fellaini 63'); Lingard (Fred 63'), Lukaku, Martial
DERBY (4-2-3-1): Carson; Bogle (Wisdom 87'), Keogh, Tomori, Forsyth; Bryson, Johnson; Bennett (Jozefzoon 77'), Mount, Wilson; Nugent (Marriott (79')