Not for the first time this season, they came up short. Not for the first time this season there is no room for excuses.
In a competition he has little regard for, United manager Sir Alex Ferguson threw everything he had at Manchester City last night. Local pride, it seems, can cause previously declared intentions to end up in the waste bin.
What a super evening this was. For City and for Manchester football as a whole. The rivalry so one-sided since the likes of Summerbee, Bell and Lee last called it on in the late 1960s is worthy of the name once more.
As much as it will irritate City fans to read it, United were the better side for much of last night's game. The football they played was more controlled. They created more chances. In some ways, they may still be favourites to progress to the final. But none of that really matters.
Look out behind you: Ferguson faces a real challenge from United's rivals City
What this game has told us more than anything that has gone before is that the gap between these teams is closing. United, for all that Ferguson denies it, have staggered backwards a little this season while their neighbours have marched purposefully forwards on the back of Middle Eastern money.
Were this not the case then Ferguson would not have fielded this side.
Goalkeeper Edwin van der Sar started his first Carling Cup game since the final against Wigan in 2006. Ryan Giggs, who we were assured was injured only on Monday, began the night as captain. Wayne Rooney despite his exertions against Burnley at the weekend was preferred to Michael Owen. Again.
After the bad feeling caused by Carlos Tevez's summer move across town and the dramatic nature of United's victory in the league back in September, City are well and truly under Ferguson's skin now. The Scot's features were dark as he strode off down the tunnel at full time.
He has lost at Eastlands too many times for his liking already. But when he reflects on this game more rationally than he would have been able to last night, it will be the manner of the defeat that will irritate him most.
In his calmer moments, Ferguson is adept at taking the emotion out of football. And when he does he will ask himself how his players have lost a game they were for a time fully in control of.
A goal to the good and with Eastlands relatively becalmed, United conceded a needless, if slightly debatable, penalty and then shipped a quite a dreadful second goal with 20 minutes left.
The gathering storm: Edwin van der Sar fails to stop Carlos Tevez's thumping penalty
Van der Sar's punch was not convincing and then Anderson got stuck under the ball as it was headed rather innocuously back into the penalty area. Ultimately, Tevez was unmarked as he headed home from a yard.
It is not a one-off, either. United have conceded goals like this all season and that is the great danger that stalks them again ahead of next week's return game. They are more than capable of scoring freely against this under-strength City team. They could have scored two or three last night.
But at the other end of the field they are vulnerable, whoever plays.
United pretended that they didn't care much about this competition. Now we know they do. What is yet to be proven is if they are good enough to stay in it.
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