Everything depends on the coach's assessment of his players, and each team has a different perspective, including the Manchester United coach, who didn't fit into his desired scenario. Although Garnacho's performance on other teams was considered excellent, every club wants players who can lift their team and comply with the coach's established rules. I think there are many things to consider in this regard.
That's true, every coach has their own different style and knows the kind of players that suits their style so I can't blame Amorin for excluding Garnacho from his team, he probably has something they he doesn't want or lacks something. We have seen so many coaches do away with the best players of some club when they take charge, Guardiola made it clear he can't work with Yaya Toure that was the best player in the team at that moment when he was appointed as manager because he can't suit his pattern. The same thing happened to Juan Mata in Chelsea when Jose Mourinho said he doesn't want Mata in his team. Also recently Xabi Alonso prefers Arda Guler to Rodrygo because of his style of play.
These things happen in football so I am not surprised. Garnacho is a talented player but if the coach doesn't want him he should go where he is needed.