Think he is a very good footballer who doesn't have a fixed position and doesn't fit into way that United play. He always reminds me of Seedorf - good technique and movement and keeps the ball moving in possession. He is a link player and the kind of player who provides the "glue" in midfield. He suits team that keeps possession and plays a fluid, short passing game. Typically, that would be as part of a 4-3-3.
He doesn't fit United's system 3-4-2-1, however. The two central midfielders need to be athletes and often competing against three opponents. He also has a tendency to get ahead of the ball, which means that midfield is wide open when team loses the ball. Fernandes is similar. None of United's midfielders suit that really.
Mainoo isn't that mobile and doesn't score or create enough to play as one of the two players behind the striker. If Amorim is staying, he is best off moving elsewhere so can play to his strengths.