Alex Iwobi struck late to hand Fulham their first Premier League win at Manchester United in 21 years.
The former Everton and Arsenal midfielder finished off a counter-attack in the seventh minute of time added on moments after Harry Maguire looked to have salvaged a point for United.
Maguire had pounced on a rebound in the 89th minute, cancelling out a 65th-minute strike from Fulham's Calvin Bassey.
United looked to be finishing the stronger as they pushed for a late winner but they were caught out by Iwobi, who secured his side's first victory at Old Trafford since 2003.
On the balance of play, it was a win Fulham deserved and a result that punctured some of the optimism brought into United this week following the completion of Sir Jim Ratcliffe's partial takeover.
United's tame performance for large part of the game underlined how much work they have to do to make ambitions of knocking Manchester City and Liverpool "off their perch" become reality.
United were slow to get going and Fulham, despite the absence of the influential Joao Palhinha through suspension, made most the running.
Andreas Pereira had their first opportunity with a weak effort which was easily dealt with by Andre Onana before Iwobi failed to make clean contact with another chance from the edge of the area.
Pereira played in Iwobi for a clearer opening on 18 minutes but he missed the target trying to guide into the bottom corner.
United survived again when Onana saved a powerful header from Rodrigo Muniz following a corner and Sasa Lukic was unable to get to the rebound.
United eventually got a sight of goal after Fulham struggled to clear a free-kick and Alejandro Garnacho attempted to curl a shot into the far corner through a crowded box but Antonee Robinson headed clear.
Diogo Dalot went even closer with a long-range effort which beat Bernd Leno but clipped the outside of the post.
Fulham responded well and hit the woodwork themselves moments later as Muniz turned Victor Lindelof in the area and thumped a shot against the upright.
The visitors turned defence into attack again just before the break, this time after Marcus Rashford, playing centrally in the absence of Rasmus Hojlund, went through on goal but took the ball too wide.
Fulham raced upfield and fed Pereira, but his curling attempt was pushed away by Onana.
In a lively end to the opening period, Garnacho then forced a save from Leno after cutting in from the left.
United suffered a blow early in the second half when Casemiro was forced off after clash of heads with Harrison Reed while manager Erik Ten Hag also removed youngster Omari Forson after a quiet full debut.
Scott McTominay and Christian Eriksen were sent on to increase the energy levels but it was Fulham who continued to dictate the pace.
Reed almost found the net when his cross-cum-shot had to be palmed over by Onana.
They capitalised from the resulting set-piece as Pereira's cross found Bassey, and he rifled his first goal for the club into the roof of the net after his initial volley was blocked.
Fulham threatened more with Reed firing narrowly over and Harry Wilson bending a chance just wide.
United roused themselves late on and Maguire reacted quickly after a Bruno Fernandes shot was saved but Iwobi had the final say.
