Sunderland boosted their Premier League survival hopes with a deserved victory at the stadium of light.
Wahbi Khazri put the strugglers ahead after three minutes with a free-kick from the left that eluded everyone.
Anthony Martial levelled after he clipped the ball over Vito Mannone, who had parried Juan Mata’s shot.
But the Black Cats won it when Lamine Kone’s header from Khazri’s corner went in via a deflection off David de Gea to dent United’s top-four bid.
Sunderland’s win takes them to within one point of rivals Newcastle and safety after a game inspired by manager Sam Allardyce’s January signings.
Tunisia forward Khazri made his first Premier League start in their last game, a 2-2 draw with Liverpool where he assisted Jermain Defoe’s equaliser.
And the signing from Bordeaux was inspired again, scoring early on against United with a free-kick which Defoe stepped over to bamboozle United’s defence and De Gea.
And their winner – while credited as a De Gea own goal – was all thanks to the powerful header from Kone, a signing from Lorient.
De Gea got a touch, with Martial appearing to hit the ball off his own keeper on the way in.
A massive boost for the Black Cats.