Honestly I'm not sure if any manager could fix the problems at Man Utd in the short term. No manager is going to be given the medium term, never mind the long term.
Man Utd have a squad of players assembled at huge expense to fix multiple short term issues. The current squad is not a good squad but it is also going to be expensive to offload the bad buys.
In order to fix the structural issues at the club it will require Man Utd to probably spend 3-4 seasons out of contention for the top 4 and probably closer to the middle of the table. Are supporters willing to accept that? Are the club owners willing to accept that?
What Arsenal have done with Arteta is similar to what Man Utd need to do.
Ten Hag has literally just been given the medium term. He was into his third season and replaced half the squad. Ironically, the players turning up and keeping him in a job were the likes of Maguire and Evans... hilarious really.
It's about progress on the pitch, you can't continue to deteriorate over this length of time, you have to show signs of improvement. That may not mean top 4, it may mean some bad performances and results, but it can't be dire every week.
Performances have been so bad that people have effectively decided we have a rubbish squad, which really isn't the case. Assuming the club continue to invest in the squad, I expect a push for 4th every season, if we fall short, so be it. Hojlund, Garnacho and Rashford are powerful runners and we have one of the best creative midfielders playing behind them in Bruno. That alone should make us more dangerous than half the teams in the league.
I think people are underestimating what a manager can actually do, such is the Ten Hag effect...