Brighton is currently in a good shape, and they always give Manchester United a tough time when they face them, thus Manchester United facing Brighton will undoubtedly not be an easy game for Manchester United. However, given that Manchester United is playing at home, perhaps they will be resilient and find a way to win the game. However, given their current form, I can't say that Manchester United can win the game against Brighton, who have won three of their first four games this season.
Old Trafford is often famous for its awesomeness and I think this time that nickname will disappear when Manchester United have to host Brighton in the match of week 5. This is not to belittle a team like Manchester United, but in fact up to now they have had great difficulty maintaining their consistency. In last season's meetings, whether home or away, Manchester United never won against Brighton. I think it's clear who the winner is this time and it's likely the score will come back with a narrow win for Brighton.