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Not really sure what's the real problem of Maguire, but I think he always play under pressure...
Every professional player is playing under pressure and they have to deal with that. Coaches and fans might accept a performance drop in a match or a two but Harry Maguire made serious mistake several times and damaged heavily the team. Sometimes you have the feeling that he is the player number 12 in ManU opponent team! I doubt he will recover and it would be better to leave the club. If he was a player outside of England, he would be kicked long time ago..
Actually the pressure comes from the players themselves, can they deal with the pressure a bit and focus more on playing so that the tick performance can be strong.
Harry Maguire is a player with a defensive position he should be able to make Man United have solid strength in defense, but everything you have said is true, that he actually played less than his maximum and made several fatal mistakes.
Maybe Erik ten Hag will put him on the bench more often next season.