United suffered only their second Premier League defeat since November as Bafetimbi Gomis' fortunate winner gave Swansea a smash and grab victory at the Liberty Stadium.
It gave the Swans an unlikely double over the luckless Reds and, whilst the defeat is disappointing, it does little to affect United's top four position with Southampton or Liverpool, or both, guaranteed to drop points on Sunday.
Gomis had scored only one league goal before this match, but the French international striker went close twice in the opening 10 minutes in a bright start from the home side.
His header from a Gylfi Sigurdsson corner was cleared off the line by Ander Herrera, and then he nodded narrowly over from a Ki Sung-Yeung flick.
The recalled Robin van Persie went close and Wayne Rooney saw his effort saved before United took the lead with an excellent goal just before the half hour.
After Luke Shaw burst down the left wing, a neat exchange of passes between Rooney and Di Maria set up Herrera, who clinically swept in his second goal in two games.
Parity was restored almost immediately though, as Jonjo Shelvey's cross was guided in from close range by Ki to pull the Swans level.
From then on, United almost totally dominated, both in terms of possession and goalscoring chances, and went close twice after half-time, through Marcos Rojo's header and Herrera's well-hit snapshot.
Rooney, van Persie and substitute Ashley Young all had chances to restore United's lead but, to sum up our day, were thwarted in the crucial moments either by a wrong bounce of the ball or last-ditch defending from Swansea.
It wasn't to be though, and the hosts were rewarded for their resilience with 15 minutes left-albeit rather fortuitously- when Shelvey's shot was heading wide until it struck Gomis and was diverted into the net.
United's day got worse when Van Persie picked up yet another injury late on and left the Liberty Stadium on crutches.
How the Reds got nothing from this game will forever be a mystery, but, with the battle for the top four still in our hands, we now need to dust ourselves down to focus on a favourable-looking run in.
United Faithful Man of the Match: Ander Herrera
Overall team performance: 8/10