He is not going to be blameless, but I think now it has become clear the players are the problem. Amorim probably wishes he has stayed with Sporting, won the league again, and could have landed a top job down the road, maybe even Man City. Or, that the hierarchy at United listened to him and he had come to the club at the end of the season.
Manager after manager and it is the same story. I remember when Klopp first went to Liverpool and we mocked his galvanizing of the squad with strange stunts. The classic new manager bounce happens when you have desire and effort. That Liverpool team was low on quality but high on effort. It took time and a hard path, but you could see that the players were willing to play for Klopp from the start. United's current team simply do not have the effort/desire and are dubious on the quality too.
They are just not willing to put in the extra running. Most fans would take the team simply looking arsed to be there. There are players there who downed tools on Ole, Tan Hag, and now with Amorim. Those players, whether it is one of ten of them are obviously impacting the dressing room and development of other players. It cannot be that every player and manager becomes shit unless there is something culturally wrong. It also cannot be that managers with a clear identity cannot impart that onto the squad when every other team in the league, even the shit ones, have a defined way of playing. The only explanation is the players are just not willing to listen to instructions. The only thing I can't figure out is why.