I think that's an overconfident opinion. It's entirely possible that neither Liverpool or Chelsea will have a coaching problem and have great results, what then? They'll be making excuses for the Manchester United coach again. If something isn't working it needs to change, you don't look at it hoping it will fix itself, that's not how it works. So replacing a broken element in the team would be a good idea
When it comes to the Manchester United, what is broken is the manager himself. He's not gotten that true and real connection with the team players, rather he just work with them as their manager is what I feel. Hence he's not able to get the motivation he needs from.each and every player he needs in his toolbox. All I advice the team to do and the club in the entirely is to sit well and have an open discussion between the players and the manager of the club so as to fix all the differences between them. This is if Eric Ten Hag is to continue as a manager. Personally, I like the guy and he's hardworking but I need him to control himself the more to become more of a sportsman than a boss to the players. Let him.be more magnanimous at heart.
To the level that I have understood football, especially the English Premier League. I can bet you that either Chelsea or Liverpool will have coaching problem if not both of them. Liverpool could be lucky in the first season not to have issues because the quality players are still around and the aura of Klopp will still be guided a club. But for Chelsea, since they refused to retain pochettino, I don't think that they can suddenly become successful this season. They could actually qualify for Europa League but they will not qualify for Champions League. How do many things can happen in new season but I am not expecting much from Chelsea because they have fallen far below their standard. So they need to gradually pick up and this will not happen eventually but through a process which Pochettino would have been allowed to complete.
For Manchester United, they need good players and ETH will actually do his best if giving good players with good squad depth.