As for their first game, I don't think they can win against Arsenal. It's probably they lost it.
United still has one friendly match scheduled against Fiorentina. Amorim has the opportunity to try out tactics and also prepare his players before the first match against Arsenal in the Premier League.
The first match will be quite difficult and tense for the two big teams in the Premier League. Although United has been experiencing a decline, they still remain a team with a great history in the Premier League. They need to work hard.