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 side, who beat them 4-0 at the Amex Stadium in May on their way to a ninth-placed finish in the Premier League, their highest in history.
Not even the second-half introduction of Cristiano Ronaldo could deny Brighton, although United 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.
“It was unnecessary to lose this game,” said Ten Hag.
“I think it was a good start and then after [the first goal] we dropped down a level, dropped down in belief and we made mistakes and the opponent punished us.
“But I think it is a good opponent, they have worked long together and when you make mistakes you get punished for that against such teams.”
