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.
Brighton can not be grouped as a weak team that wins against Manchester United. Brighton has won against the big and small teams in the past; they won't stop doing that. For Brighton to win against Manchester United doesn't make them better than Manchester United or will end up better than Manchester United at the end of the season. Shit like this does happen in a match, for the anticipated club to lose against their lower opponent. However, it was another lucky day for Brighton to have another fantastic win in their second match of the Premier League season. They won't keep on having good wins like this, same as Manchester United having a loss match all the time.