Sunderland boosted their top flight survival hopes with a late and deserved victory over United to surely end any lingering top four hopes for Louis van Gaal and the Reds.
Wahbi Khazri- on his home debut- put Sam Allardyce's strugglers ahead after three minutes with a free-kick from the left that evaded everyone on its way in.
Anthony Martial cleverly lobbed United level but the Black Cats won it when Lamine Kone's header went in via a deflection with eight minutes to play.
United headed into this clash unbeaten in 13 league visits to the Stadium of Light and with the top four all facing off on Sunday, it was an opportunity to close the gap but, instead, surely now our only way into the Champions League is through the Europa League back door.
Having fallen behind to that Khazri set-piece, the Reds almost went two behind soon after through livewire Jermain Defoe whom was thwarted by a good block from the stretching Chris Smalling.
Khazri lashed over the bar but that seemed to lift the Reds from their slumber and, after a succession of corners, Wayne Rooney picked out Juan Mata whom fired straight at Vito Mannone when well-placed.
Somewhat appropriately on Valentine's weekend, United had D.Love soon after when Matteo Darmian went off injured and 21-year-old Donald came on for his Reds debut at right-back.
United's equaliser immediately followed when Mata's driven effort was pushed out by Mannone, but the rebound fell to Martial whom delicately dinked the ball over the Sunderland stopper five minutes before the break.
David De Gea saved from Dame N'Doye with his feet, Daley Blind kept out Defoe with a brilliant lunging tackle, and the dangerous Khazri went close from another set-piece.
Kone forced an acrobatic save from De Gea, whilst at the other end Mata fired straight at Mannone and then shot just off target from birthday boy Memphis' dangerous delivery.
The Reds continued to probe for the winner as Smalling latched on to Rooney's deep cross but headed narrowly wide, but Sunderland would not waste their opportunity and struck on 82 minutes.
Two of Allardyce's January signings combined for the winner when Kone met a Khazri corner with his head, the ball struck De Gea and crept over the line despite the brave attempts of Smalling to clear.
The aim was now to salvage a point but, despite a smart stop from Mannone to keep out Memphis in a late rally, United were to slip to a seventh league loss of the season.
United Faithful Man of the Match: Anthony Martial