I still can't say for sure that will happen, but Mount has had a difficult time with Chelsea and has been under an incompetent coach. Currently he has not shown the maximum performance we expect. Pre-Season wasn't very exciting to watch, maybe for the Red Devils fans, something that was important to see Mason Mount's first game in a Manchester United shirt.
Mason Mount had a very good time in Chelsea. He graduated to the senior team from the Academy under Frank Lampard and from then he has had a good time in Chelsea. He had his best Chelsea experience under Lampard and it extended to the time of Thomas Tuchel. He played his best football under the German tacticians and he won some of his current titles. He won the Champions League, UEFA Super Cup, Premier League, and FIFA world club with Chelsea.
Mount had his bad time just last season under Graham Potter. His last season was also marred by injuries which made him fall out of favor with the coach. The second coming of his benefactor Lampard didn't yield much result because he has already started having contract issues with the club. For me, I believe that he had a successful time in Chelsea. It is also too early to start judging the performance of Mount. He is still young and can perform for his new club.