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.
I also think Ganacho is a good player, but I realize I'm not management or a coach. So I think the Man United coach and management want greater expectations from Garnacho at the moment. But during Amorim's time there that didn't seem to be met. So Amorim is preparing to sell him to get a player they think is better. This seems to be a situation we have to accept, although I think there are many factors in the success of players in the team including the coach himself. So Amorim must indeed be prepared to accept if indeed Garnancho performs better with other clubs.