David Moyes ended this dire defeat with his arms folded, entering and exiting the technical area in the vain hope that his Manchester United side could somehow answer Wilfried Bony's late strike.
The tie had entered a 90th minute when Wayne Routledge zipped past Darren Fletcher down the left and popped up a ball that the unmarked Bony headed beyond Anders Lindegaard.
At the close Moyes was ashen-faced and the unpalatable facts read that this was a fourth home defeat in a month, a fifth in total. With the manager and the United hierarchy contemplating a January transfer window in which little or no strengthening may occur, where the Moyes project goes from this juncture will fascinate.
The Scot admitted a pressing need to recruit. "There is an urgency that we would like to bring people in, but are those players available in January? So there is no point in me hyping it up because the players we would like to bring in are probably not available in January, not because we don't want to do it," he stated. "I said I would try but probably would be doubtful in January, because of the window."
Despite having seven players injured, including Wayne Rooney and Robin van Persie, the manager refused to use this as a mitigating factor. "It shouldn't be that [being too reliant on Rooney and Van Persie]. We put out a really strong team, everyone is getting an opportunity to show what they can do and, hopefully, then I will make the decisions from that.
"No excuse – just about every player we had out was an international. Maybe Alex Buttner wasn't, but the others are internationals."
The midfielder Fletcher continued the honesty theme. "It's a massive blow. Let's not kid ourselves here, this is a really bad result, and there are a lot of angry, hurt players," he said. "And we feel we have let the manager and the fans and everyone down today by losing."
As this contest entered the final 20 minutes the old chant of "attack, attack, attack" could be heard from the United congregation. The problem is that while the champions can still dominate for periods they lack that telling edge. Here, again, the issue was paramount against a Swansea City side that offered little yet still came away with the victory and should be applauded for having the X-factor that United so lacked.
The visiting manager, Michael Laudrup, was polite in his assessment of Moyes's team. "I'm not one to sit here and say what level United compares to Barcelona, Madrid, Bayern Munich, Chelsea, Man City," he said. "In my world they are still a big team."
As with the 2-1 defeat against Tottenham Hotspur on New Year's Day, United ended the contest camped outside the opposition area but were too blunt and predictable in what, at times, aped a training-ground game of attack versus defence.
For the manager the sight of Rio Ferdinand, making a first appearance for a month, limping off towards the end and his replacement, Fábio da Silva, lasting four minutes before receiving a straight red card for a challenge on José Cañas was yet more attritional fare to digest.
Swansea opened the scoring when, from the centre circle, Bony bounced the ball back to Alejandro Pozuelo and a first-time pass split the United rearguard. Routledge moved on to the ball, held off an exposed Chris Smalling, and lobbed Lindegaard expertly.
Less than four minutes were required for Javier Hernández to square proceedings. His fifth goal of the season was a simple affair, Buttner thumping in a cross from the left that the Mexican finished first time beyond Gerhard Tremmel.
But this failed to steady a United side showing six changes from the 2-1 defeat to Tottenham. If Moyes informed his team to wake up during the break they failed to do so. Almost instantly after the restart Pozuelo was allowed to dance forward before unloading an effort in what proved an augury of Swansea's grandstand finish.
Moyes has now overseen an FA Cup third-round exit at his first attempt as United manager, equalling the grand total Sir Alex Ferguson managed in nearly 27 years as his predecessor.
The Scot needs Rooney and Van Persie back soon. But as with the uncertainty over the precise injury that Ferdinand suffered, the manager said he was unsure if one, or both, of his frontline forwards would be available for Sunderland.
Man of the match Wilfried Bony (Swansea City)