The Red Devils made a bright start to the campaign with a home win against Fulham on the opening day, but their joy was immediately ruined after Joao Pedro's 95th-minute winner in the lunchtime kick-off.
Amad Diallo had cancelled out Danny Welbeck's goal before Alejandro Garnacho's goal was ruled out by VAR for offside against Joshua Zirkzee, but United were easily second-best as the hosts dominated the play.
Brighton cranked up the pressure late on and got their reward after Pedro exploited some awful marking at the far post to head home and steal all three points.
It was almost inevitable too given the lack of organisation in the United backline, with three Brighton players unmarked at the far post and Pedro took full advantage to head home the winner.
Ten Hag has spent £112million on three new defensive signings this summer, with Noussair Mazraoui, Matthijs de Ligt and Leny Yoro all arriving. However, Mazraoui was the only one to start the game at the Amex Stadium, with De Ligt on the bench and Yoro sidelined with injury.
Joao Pedro scored a 95th-minute winner for Brighton (Image: Getty)
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De Ligt did replace Harry Maguire late in the game and the lack of leadership in organising the backline was palpable as United crumbled in the final moments.
It was a familiar sinking feeling for United fans, who watched their side concede 20 goals in the 80th minute and beyond last season. They are course to beat that total, with two conceded beyond that mark this season already in two games.
According to Opta, Manchester United have lost six games to goals scored in the 90th minute and after in stoppage time since the 2022-23 campaign - more than any other side. In the previous 30 seasons, they had only lost two games in the same manner.
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Manchester United fell to their first defeat of the season (Image: Getty)
6 - Since 2022-23, Manchester United have lost more Premier League games thanks to goals scored in the 90th minute (+ stoppage time) than any other side (6); they'd only lost two such games in the first 30 campaigns in the competition combined. Asleep. #BHAMUNpic.twitter.com/F6JXEWUHti
Speaking after the game, Ten Hag made clear his disappointment with his team's inability to hold on for a point and identified where his players failed in their duties.
“We didn’t stop the cross. There were three players, [Adingra is a] right-footed player. Send him down the line, send him wide instead of letting him come in and let the cross," Ten Hag told TNT Sports.
“We have of course to talk about this, how we act in that situation as a team. There were more than one mistake on that occasion and it has a big impact on the score.
“That [conceding the goal]’s very disappointing, yeah. Because in the Community Shield we concede just before full time, we should be more clinical in such situations, read the game, take the point if you can’t win any more. We were close to winning this game but definitely don’t lose the game.”
