A brace from Pascal Gross in the first half gave Brighton a comfortable lead until 68 minutes when Alexis Mac Allister scored an own goal. However, United were unable to convert any chances, giving Brighton their first-ever win at Old Trafford.
- Premier League team-by-team guide and burning questions
- O'Hanlon: Ranking the Premier League's best players (E+)
- Premier League kit ranking: Which jerseys are 2022-23's best?
- Dawson: Man Utd show familiar flaws on Ten Hag's debut
"We went into the game positive, we had a good preseason and it was the worst possible start to be 2-0 down at half-time at Old Trafford. Not good enough. We got a goal back but we huffed and puffed a bit, and it's a bad start for us," Harry Maguire said.
"We started pretty the game well and on the front foot and as a team we had control of the game. Then we conceded the first goal and that knocked us badly. We stopped playing from then.
"We need to look at not letting that first goal set us back as much as it did. But we have to be better on the ball, we gave them encouragement to come forward and attack us. It was a nightmare start to be 2-0 down at half-time."
It took Brighton 30 minutes to break United in a spell where they were putting on most of the pressure. Danny Welbeck got away from Maguire and hit the ball into the six-yard box where Gross tapped it into the net.
His second came nine minutes later in a well-built team goal from Brighton. They worked the ball down the field from their corner flag where Solly March picked up the ball on the edge of the area.
He took a shot, which was palmed away by David de Gea, but only as far as Gross, who stuck the ball into the roof of the net.
Instead, it was a poor decision from Robert Sanchez -- who had made some fine saves -- which saw Mac Allister guide the ball into his own net in a goalmouth scramble.
The goalkeeper had failed to clear the ball from the corner, and as it bobbled towards his net a second time, he hit the ball off Mac Allister and into his net.
VAR checked if there was a handball from Diogo Dalot in the buildup to the goal, but no foul was found and the score was given.
"It was a great performance and a deserved win. A great way to start," Brighton coach Graham Potter said.
"I thought we had an idea of how to attack and when we could we tried to press and put them under pressure. We were positive, brave and the boys gave everything. Our performance in the first-half was really strong."