So you assume Mason Mount did not grow well under an incompetent coach (Thomas Tuchel and Graham Potter). So I will look forward to his debut with Manchester United under coach Erik ten Hag.
But if he is still in the same situation as he was at Chelsea, then it is not Tuchel and Potter who are incompetent, but the players themselves who are not so talented.
I think what Mason Mount is going through is similar to what Marcus Rashford went through the other season at Old Trafford. Consecutive change in manager has affected his game play coupled with his poor form in the league. I believe Ten Hag is able to get the midfielder firing on all cylinders in the new season.
Remember; it is difficult to perform well; if the entire squad is struggling.