Even changing coaches too often can sometimes have a negative impact on a team experiencing setbacks. A new coach will inevitably bring new strategies and coaching styles. Adaptation will occur, both for established players and new arrivals. None of this will be easy. And the results won't be as immediate as imagined. It will take time. But if the current management can solve this team's problems quickly, I'd truly commend them. However, I doubt Manchester United will be able to bounce back immediately at the start of the season. The team's chances of improving are likely to be towards the midpoint of the season. Amorim is a great coach, but he's usually the type of coach who needs time to maximize team performance, as he demonstrated at his previous club, Sporting CP.
They will not bounce back ever. I mean even if they do slightly bit better, we all know that they are not going to end up doing that much better, because the reality is simple, the management sucks and they keep getting bad players and they keep having bad "non-game" related stuff. Basically everything that is outside of the field is wrong for them.
I remember clearly when Ronaldo mentioned like the team was exactly the same since he left when he came back, the machines and all that, literally gym and recovery and all that. So they made a point of making sure that they are seen as doing something better at least. But at this point, I do not believe they will do any better for the fact that they are not managed properly at all. As long as this happens, we are not going to see anything change all that much.