Joao Pedro headed in a 95th-minute winner on Brighton manager Fabian Hurzeler’s home debut to inflict a 2-1 defeat over Manchester United after Joshua Zirkzee had inadvertently stopped the visitors from taking the lead during the second half.
Zirkzee - the 23-year-old summer signing who won United the points against Fulham last week with a goal on debut - denied team-mate Alejandro Garnacho from slotting home in the 70th minute at the Amex Stadium by getting the final touch on the goal line in an offside position.
"We played the overload very well," said Ten Hag on the disallowed goal. "Then it's about bringing the final pass, bringing the players in the right positions, getting the right finish. We did everything well there. It's very misfortunate."
Hurzeler admitted: "It was lucky that we didn't concede the second goal."
- As it happened: How Joshua Zirkzee denied Man Utd
- Teams | Match stats
- Live Premier League table | Watch PL highlights for free
- Stream the Premier League with NOW
United had edged the first half but had to come from behind when their former striker Danny Welbeck poked Brighton ahead in the 32nd minute from inside the six-yard box.
Marcus Rashford appeared to have grabbed an instant response but had his effort ruled out for offside and although Zirkzee was brought on at half-time for the injured Mason Mount, it initially did little to help United's search for an equaliser.
Instead, Brighton had a James Milner prod cleared off the line by Diogo Dalot before Welbeck headed against the crossbar. The woodwork was still shaking when Amad Diallo raced up the other end and fired home via a deflection off the sliding Jan Paul van Hecke just 100 seconds later.
United pushed for a winner and thought they'd got it when Garnacho netted but Zirkzee, who had slid in to meet Bruno Fernandes' low cross, made contact with his knee before the ball had crossed the line.
And, typically, Erik ten Hag's side were made to pay in injury time when Simon Adingra tossed a cross into the box for the unmarked Pedro to head in as Brighton players queued up at the back post to earn their second victory of the season.
How Zirkzee denied Garnacho
Ten Hag: We should take a point
Manchester United manager Erik ten Hag on conceding late goals:
"That's very disappointing because also in the Community Shield we concede just before time so we should be more clinical in such situations, read the game, take the point.
"If you can't win anymore, and I think we were close to winning this game, then definitely don't lose the game.
"We should keep more control in the game but I think there were very good parts. I've seen a lot of positives from the performance but we have to build up in both boxes. We have to be more clinical there, we have to be more aggressive."
On Mason Mount being subbed at half time: "It was forced. He mentioned an issue and I don't want to take the risk with him.
"I don't know how much of a worry it is, I can't say at this moment. But I think, when we have the good bench, and we can bring in Zirkzee, then don't hesitate. Bring Mount off and bring [on] a player who can have an impact."
Hurzeler: The players never gave up
Brighton manager Fabian Hurzeler:
"My team deserved to win. After we conceded the goal, we lost a little bit of control of the game. The game was too open and that's one of the biggest strengths from United, to have good transition moments.
"How we played in the last 10-15 minutes was amazing. We were pushing for the second goal, we were controlling the game, we had chances.
"I'm very happy that the players had the attitude of never giving up and always believing in their own strengths and believing in each other. I could feel it on the sidelines."
United show their weak away-day mentality again
Sky Sports' David Richardson:
Manchester United are naive. Since 2022-23, they have lost more Premier League games thanks to goals scored in the 90th minute and beyond than any other side (6); they'd only lost two such games in the first 30 campaigns in the competition combined.
Already this season, they conceded an 89th-minute equaliser to Manchester City in the Community Shield.
The momentum at the Amex had swung in the final 15 minutes. United and Erik ten Hag failed to recognise it. Scott McTominay was brought on for Bruno Fernandes rather than Casemiro, whose legs had faded.
Harry Maguire was needlessly changed for Matthijs de Ligt and United missed his aerial presence at the corner which led to Brighton's winner. Ten Hag insisted his side "take the point" in that scenario but neither he or his players tried to.
United are winless in their last 14 Premier League away games in which they've conceded first (D2 L12), since coming from behind at Everton in October 2022 (2-1) and that is coupled with Ten Hag's miserable away record which has more defeats (17) than wins (16) in the Premier League.
When United come under pressure on the road, they crumble and on this evidence, more pain will follow.