Brighton survived a late onslaught to secure a first win at Old Trafford and condemn Erik ten Hag to a losing start as Manchester United manager.
Pascal Gross’ first-half double stunned the home supporters, who were desperate to see their team put a miserable end to last season behind them.
However, United were totally outplayed in that opening period by Graham Potter’s team, who beat them 4-0 at the Amex Stadium in May on their way to a ninth-placed finish, their highest in the top flight.
Not even the second-half introduction of Cristiano Ronaldo could deny Brighton, although they did threaten a comeback when a scramble inside the visitors box ended with Alexis Mac Allister turning the ball into his own net after goalkeeper Robert Sanchez had touched it into his path under pressure from Harry Maguire.
In nine previous league visits, Brighton had managed only one draw, so this victory was historic.
For Ten Hag, it meant enduring a similar start as United manager as fellow Dutchman Louis van Gaal, who lost his first game by the same score, also at home, to Swansea in 2014.
With the sun shining and a new manager in place, there was a sense of optimism around United before kick-off.
Brighton, well drilled and fluid, took advantage. Leandro Trossard volleyed into the side-netting within 15 seconds of the start and United failed to impose themselves on the game until it was far too late.
For the opener, Welbeck’s run split Maguire and Martinez. The latter came across from the left to cover the cross, leaving a huge hole in the six-yard box and, as United’s defence scrambled to plug the gaps, Gross was on his own to tap home at the far post.
Gross was in a similar position but on the other post when Solly March cut inside and unleashed a low shot, which keeper David de Gea turned into his path.
The boos that accompanied the half-time whistle were predictable and deserved. Within eight minutes of the resumption, Ronaldo had been introduced to try and lead a rescue mission.
The 37-year-old had a negligible impact. It was another Portuguese, Diogo Dalot, who created the confusion that led to United’s goal. But by the end it was Brighton who looked more likely to score.
Man United 1-2 Brighton
Everton 0-1 Chelsea
Leicester 2-2 Brentford
Fulham 2-2 Liverpool
Tottenham 4-1 Southampton
Newcastle 2-0 Nott Forest
Leeds 2-1 Wolves
Bournemouth 2-0 Aston Villa
Crystal Palace 0-2 Arsenal – BBC