The defense line was still quite problematic in the match so they had to accept defeat. Even Brighton's winning goal came from Man United's poor defense because they did not guard the player who was standing empty in the corner. As far as I have seen in the last five meetings between the two, Man United have only been able to win one match and this is a pretty bad result for Erik Ten Hak.
Brighton was able to match Man United's game and I think this is a weak point that needs to be fixed by Erik Ten Hak. If they lose against a team like this, it will be much more difficult when facing other strong teams in other matches.
Yeah, the defense problem is still the main problem for Manchester United, at first I thought they were safe when they equalized the score but disaster came in the final minutes. As you said, Ten Hag must find a solution if they want to stay in the top 4 morever this season competition is getting tighter, Manchester United still have time to improve their defence in this transfer window just wait what they will do.