This is a sign of frustration from Amorim. Didn't he say he wouldn't change his formation, loudly declaring it all depended on him? Now, a change in formation signals Amorim surrender and a gamble. Like it or not, Sunderland comes into the Premier League this season with incredible motivation, so I wouldn't be surprised if Manchester United lose. Sunderland six matches have 3W, 2D, and 1L, making them quite optimistic for a visit to Old Trafford.
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.