•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.
•In nine previous league visits, Brighton had managed only one draw, so this victory was historic.
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.
ForFor 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.
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.
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.