In the absence of talismanic figurehead Zlatan Ibrahimovic at Chelsea, United needed world record £89 million signing Paul Pogba to step up and prove his worth.
When a player signed for an over-the-odds fee is struggling for form and his team have lost, he is always going to be made a scapegoat and Pogba is no exception.
The spotlight will always fall on him, sometimes unfairly, but the Frenchman does not help himself with his frequent change of hairstyles and shameless flogging of "Brand Pogba."
The effort and endeavour of many of United's players put Pogba to shame.
Marcus Rashford ran himself into the ground despite illness and Antonio Valencia, Ashley Young and Marcos Rojo - late clash with Eden Hazard aside - all enjoyed more profitable nights than the fitful contribution of the world's most expensive player.
He struggled again and another disappointing night was amplified by the relentless brilliance of the hosts magnificent midfield metronome N'golo Kante.
The contrast between the two Frenchmen could not have been starker.
Where Pogba's showing was one of listless indifference, countryman Kante was once again quite simply immense for his side.
Kante was perpetual energy and industry as he started attacks, broke up United's moves, linked the play effortlessly and capped another man of the match showing with a brilliant and deserved winner.
Pogba simply could not get into the game either before Chelsea took control or afterwards when Mourinho looked to his showpiece summer capture - the signing he set his heart on - to revive United.
Pogba's quality should prevail given time at Old Trafford but he is falling short at the moment.
He was not signed for his defensive acumen but is not offering nearly enough going forward either.
When an opportunity did arise in front of goal, he fluffed his lines with a presentable chance that quite possibly would have forced extra-time.
Chelsea's fans revelled in Pogba's struggles as they serenaded him with chants of "what a waste of money" - accusations that cannot be levelled at Kante who looks a £30 million bargain.
In his defence, he looks like a player who's been running on empty of late and is in need of the rest Jose seems averse to giving him.
Fatigue seems to be compounding his problems but he will probably start on Thursday and in the upcoming league games with Middlesbrough and West Brom because of Ander Herrera's suspension.