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.
Both players didn't meet our expectations, the potential is there but they would be better somewhere else, they are both in their early peak and might do well or even worse but that's left for Arsenal and Manutd fans to worry about now, I've seen a lot from both players and I'm not expecting anything more from them at their clubs. Both of them contributed to some of our problems last season, and I'm delighted they're gone; they were not the type of players we want; Kai is sluggish, and Mount was injured for the duration of the season and was nothing near his best.
Personally I think Mount would thrive anywhere he plays.. Kai though might need certain environments and attention for us to see him at one of the best forms!!! But time will tell anyways!!