Fulham earned first Premier League win at Man Utd in 21 years. Calvin Bassey opened scoring before Harry Maguire netted late leveller.
Alex Iwobi hit 97th-minute winner to leave Man Utd eight points behind fourth-placed Aston Villa.
United had been in superb form in 2024, remaining unbeaten and winning their last five matches, while Rasmus Hojlund had scored seven goals in his last six games.
But the striker was ruled out through injury in midweek and United were toothless in his absence, with Omari Forson – handed a first senior start after being preferred to £86m signing Antony – struggling to fill the void.
Fulham wasted numerous openings and the only surprise was that it took them until the 65th minute to open the scoring, with Calvin Bassey lashing the ball into the roof of the net after United failed to deal with a set-piece.
The hosts belatedly mustered a response and levelled when Harry Maguire tapped in from close range after Bernd Leno spilled a shot in his direction.
That goal swung the momentum in United’s favour but Fulham hit their hosts with a sucker punch, Iwobi finishing a 97th-minute counter-attack to leave Ten Hag’s team eight points behind fourth-placed Aston Villa.
The lose has also dented United’s chances of ever participating in next edition of UEFA Champions League tournament.