You should already know that Arsenal is not an easy team, even before this season. Both teams have new signings, and the signings at Arsenal are to boost the team's performance, while those of Manchester United aim to revive the team's performance from last season's poor results to a better one.
So I think Arsenal has better chances of defeating Manchester United. I won't use Manchester United's performance against Arsenal to conclude what their performance will be like this season; there are more games to see if there will be a change this season.
Although in terms of the match that will take place, I still think Arsenal has a pretty good defense after seeing it during last season's match in the Premier League, and that has been proven by their performance which is quite ready when they will face Manchester United, while Manchester United, although there has been an improvement when competing in the pre-season, must pay attention to defense, maybe that is what should be focused on now.