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.
Indeed, it is the job of a coach to make the players on the team he coaches better every season, especially for new players like Mason Mount who will defend Manchester United next season. Of course Ten Hag will make him a more useful player in the team to improve Manchester United's strength next season because next season Manchester United will also be competing again in the UCL where the team really needs the best players to be able to face their rivals.