I don't agree that ETH should be fired from Manchester United. In my opinion, the worst situation at Manchester United recently is not only the coach, but all the players must be responsible for their own play.
I don't think the players are the problem of the team; for this case to keep arising with different players, I think the manager needs to work on himself on how to relate with players so that he can get the best in all the players in the team.
When he came to United, the atmosphere was well ok and the players were happy with each other, especially as they had Ronaldo back in the team. ETH succeeded in kicking Ronaldo out of the team, and also David Degea, who is now having problems with Sancho, who is expected to be active in the team when they have injury problems.
So if I look at Manchester United as it is now, it is more or less the same as what Liverpool experienced last season because of the storm of injuries at Manchester United. The players were also exhausted last season from playing 4 leagues and a World Cup which made the team's schedule worse until Manchester United played eight times last February (this is just an example). Manchester United just needs time to recover, so they have to give ETH the opportunity to build a strong squad.
They played in four competitions just like Manchester City, but Manchester City did not complain. Although I agree with you that injuries affected their performance, that should not be an excuse for the team's lack of form. I think Ten Hag needs to put in more effort to improve the team performance, and if they give him time, he will settle in at Manchester United and bring glory back to United.