Changing strategy is far better than sticking with a back three and always losing, and it would be a good thing if Amorim seriously considered his new formation. He is indeed a bit stubborn, as in several interviews he has consistently stated he won't change his formation. Conceding goals is nothing more than the defense's difficulty in blocking the opponent's attacks, and that is indeed Man United's weakness so far, which Amorim needs to address immediately.
Sunderland are not a tough opponent and they are motivated after achieving promotion to the Premier League and teams like this usually have no burden in playing. So if Amorim still maintains his ego by using three defenders, it will not be surprising if Man United later suffers another defeat.
Amorin should try something new about the formation and change it for the good of his team, because in controlling the ball the defensive midfielder is considered less solid and that is what needs to be fixed now when I pay attention to his shortcomings. Even though Amorin has become accustomed to his three central defenders with the formation of the number of defenders being four, it will build a more stable strength when the opponent counterattacks.
I don't think Amorin changing formation is the best, because he has enough time throughout last season till this new season nothing new from him or change from the club, Manchester United needs a new coach rn to understand the faults in the team and then try something new and make changing to the club, Manchester United isn't longer the Manchester United we use to know before again, no improvements from the coach Amorin, so they need a new coach to elevate the club and bring back the United we use to know before, I don't think Amorin is the best coach for United, they should bring a new coach that understands the game more better, Manchester United is a great club so it needs a great manager/ coach to make it great again.