Brighton capitalised on a disastrous mistake from Manchester United goalkeeper Andre Onana to condemn their hosts to a fourth defeat in five Premier League home games and leave Ruben Amorim’s side rooted in the bottom half of the table.
The visitors were already leading thanks to goals from Yankuba Minteh and Kauro Mitoma when Onana came sliding out to collect a low Yasin Ayari cross.
It seemed an easy enough take but somehow Onana managed to grab the ball, then release it. Substitute Georginio Rutter could not believe his luck as he turned the ball into an empty net, with Onana left exasperated at his inexplicable failure.
It was hardly the way United’s players would have wanted to mark the death of legendary striker Denis Law, but they deserved nothing more.
With world darts champion and lifelong United fan Luke Littler watching on, Brighton struck after just five minutes as Mitoma scorched away before presenting Minteh with the inviting finish.
Then, after Bruno Fernandes had pulled United level with a 23rd-minute penalty, Minteh turned provider on the hour as he chipped a cross to the far post and Mitoma won a duel with Noussair Mazraoui to turn the ball into the net from less than a yard.
By that point, Brighton had already had a Joao Pedro effort ruled out by VAR because of a foul in the build-up on Diogo Dalot.
It was the Seagulls’ sixth win in seven meetings with United, while Manchester City are the only club to have emulated their achievement of winning three games on the trot at Old Trafford in the Premier League era.
While the result keeps Fabian Hurzeler’s team in touch with the contenders for European qualification, United’s three-game unbeaten run is at an end. They have now lost four of their last six league matches and are seven points adrift of 10th-placed Fulham and 10 ahead of the relegation places.