Mount last season with Chelsea was not so good. in fact he only made 3 goals in one season in the Premier League. We know Chelsea last season was in poor form, but we don't know how he will develop at MU.
Chelsea was in a bad form last season and it affected the mentality of all the players in the team, bot the players who have been in the team and seen better seasons and the players who were brought to the club last season. The players who have seen better seasons with the club like Mason Mount, Hazertz struggled all season, and the new players who joined struggled to play well in this environment that had no winning mentality. We will see players like Havertz and Mount play better again at the levels which we know they can perform in this new clubs that they have moved to.