Definitely, although if a team is not in good form, everybody blames the manager, especially the fans. They feel like the manager is always the one who makes the wrong decision towards the team, which is not so. As for Manchester United's current problem, I will say the players are not good; they have many weak positions, and as you said, some players are not playing their positions the way they are supposed to. The management needs to get new players to make them strong again ahead of next season to see how good Amorim is; the manager can not show his talent with weak players.
A coach decide who play on the field, the positioning and every decision each player makes whether it's their own decisions or his guide, he has every reason to be called out if his boys aren't doing as they should. Players are only professional in handling tasks, how to communicate with balls but the way and manner they do it is basically directed by the coach and that's why you see fans blame the coache when the team isn't doing well, that's his responsibility.
Even if the players are bad, he will receive all the blame from people because he gets the money on him and he signed the players that are playing for the team. So, if the players he signed aren't doing well, he will receive the bash from people that aren't happy about his performance. Sometimes, you see caoch make mistake and they are obvious but they don't listen to criticism, they only follow their judgment and if you follow your way, you will be bash when your way isn't bringing wins.