If the English Premier League was as easy as the French League, it would have been easy for Manchester United to make it to the top five with their current team, but the English Premier League is always difficult compared to other leagues, so to make it to the top five, you have to perform well. There is nothing to blame the manager for Manchester United's poor performance last season because this manager did not get a good team to manage. When a team has good players and a team is formed with the best players in every position, if a manager cannot do well with that team, then it is the manager's failure, but Ruben Amorim finished the season with a haphazard team and he should be thanked for the good work the team has done under him. Manchester United qualified for the European League in the previous season as well, but they could not qualify for the European League final that season, but in the last season, Manchester United qualified for the final under this manager.
I think the problem of Manchester united is both the coach and the players in a way, you can't coach a club without efficient and reliable players who you think can win matches for you, the first error came from the coach by not drafting out list of players the club should get for him, secondly I really don't know what is wrong with Manchester united players performing very badly, except from Bruno and Amad i don't see who is performing well in that team, the best advice for them right now is to keep signing. Talking about them making it to the European cup last season was the help of Bruno (mostly him), but i think this season they will perform better than last season through the help of their new signings, but they might not make the top five list next season, if they fail to improve.
It's distinct from club to club whether the coaches have the right to choose players they want in their team. I don't know how Manchester United handles this, but I think Amorim has something to say there. One of the biggest problems for Manchester United now is that they did not qualify for the Champions League and that makes a big difference for most of the best players when they have to decide where they want to go next. It is not happening very often that some of the best players are willing to play a season without Champions League or Europa League. Additionally they don't know whether it is only for one season. If it doesn't go very well in the next season, it could be years for those players without the Champions League.