The support from Manchester United's management has always been very strong. Especially remember the times under Ten Hag, when they bought a huge number of players and basically rebuilt the entire team, but it didn’t bring the expected results, which is why he was sacked. With Amorim, things are a bit different he’s only just now, in the summer, starting to buy some players. But I agree that this is not enough to really change anything in the team.
That isn't really a "support" from management, that is literal definition of mismanagement from them. They have spent way too much on players that do not deserve it and then the managers were bad too. There is no reason to assume that managers are a big part of this, sure they were bad, but isn't the point of management to get a good manager too? Not like they are getting Xabi or Ancelotti or Pep or whatever, they are getting Amorim, ETH, and what not.
Amorim has shown that he wasn't ready for United, at first people said it's early games, and then people said it was not his team, what will be this years excuse? That he took over a dying team? I guarantee you, he is a bad manager there too. If all players become bad there, and if all managers become bad there, then maybe the trouble is the management?