Joao Pedro was left totally unmarked in the 95th-minute to snatch a 2-1 win even though United had a total of 10 players defending in their own penalty box.
'There were so many things in that moment. We have to talk and be aligned in such moments about what to do. Everyone knows their job and as a team we protect our goal.
'We didn't stop the cross (from Simon Adingra). There were three players, right-footed players, who should have sent him down the line and not allowed the cross.
The goal triggered ecstatic scenes and condemned United to defeat in their second game of the season
'We have to talk about how we act in that situation as a team. There was more than one mistake on that occasion and that had a big impact on the score.'
United's defending was also poor for Brighton's first goal by Danny Welbeck. 'We conceded two soft goals where we should act better as a team,' added Ten Hag. 'We got two unnecessary goals, very avoidable.'
'In the end you have no points, and that is disappointing because it could have been very different.'