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.
In addition to the defense line that still needs to be improved properly by Ten Hag this season, I see the midfield also needs to be modified a little with more suitable and better players so that they can help the current front line players at Manchester United. I also thought that Manchester United would not lose that match, but Brighton has a complete squad that is too strong so that they can continue to attack with good tactics until the final minutes of the match and as a result Brighton can win at home.