If he still wants to be at Manchester United for a long time, then he must immediately fix the problems in the club, be it in the game or things outside of it, such as harmonization between players and so on. What I see from Manchester United now is that they don't seem to have good harmonization. Because however in a club it must be created and will later affect their chemistry on the field.
Since Ten Hag joined Manchester United, I see there are always personal problems, and the biggest is his dispute with Ronaldo.
It seems like Erik ten Hag has a poor way of building harmony in the team so yes, the result was a collapse because he didn't manage to build good chemistry in the squad. I think, the main thing that a coach should do in building a team is about prioritizing harmonization, and Erik ten Hag does not have good skills in this matter. Therefore, then of course Erik ten Hag will not last long at Manchester United, because of course Manchester United also wants to have good harmony of course to support the team's performance to be even better.
Exactly as I said before, indirectly the harmonization within a club will be one of the factors whether they perform well or not. This is actually not only the job of a coach, but in this case the coach will certainly be the first person who has to create it all. Why? because he is the closest person to the players, and he is also the person who can communicate directly with management. thus, a coach should be able to bridge the two important sectors in a club. This should be more attention given by the management, they should also see the coach in terms of their togetherness with the players.