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.
New season, same problems for Man Utd
With the sun shining and a new manager in place, there was a sense of optimism around United before kick-off.
Ten Hag has started to impose his personality on the club, to the extent of ousting some of the ‘prawn-sandwich brigade’ from their corporate hospitality suites so his players could have their pre-match meal.
Yet so many of the old issues remained. Protests against the club’s owners before kick-off meant the club shop had to be shut.
Co-chairman Avie Glazer was paying a rare visit and would have heard the songs to his left calling for his family to get out of the club.
On the pitch, there was no cohesion despite decent debuts from Christian Eriksen and Lisandro Martinez.
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 again.