Considering the duration Ruben Amorim have spent at Old Trafford as Manchester United manager, I strongly disagree with your opinion that he's the club's worst manager. He might not be as consistent as most of us predicted at the start of the season but he's not as bad as you're trying to make him look. Manchester United are I believe will do better under him next season when he must have signed players of his choice during the summer.
It's also true that we haven't seen much improvement in Amorim's performance since he joined Old Trafford as Manchester United manager. We had full faith that he would be able to do well if he took over at Old Trafford and that Man U would find a new rhythm but this team could not move forward that way. Let's see how much better he can do for this team.
But I would say that Manchester United have been temporarily able to improve, watch their performance in the Europa League. But if everyone now blames Manchester United manager, then it may be selfishness, now the players of their team should be blamed a lot. So it has to be said that Manchester United should improve further.