They did lose a lot of players in the transfer market, but to me, it's like changing your wardrobe. When you have clothing that are no longer useful to you, you should get rid of them, and that's what Chelsea is doing right now. There are many extra players on the team who no longer contribute much to its success. They must get rid of the excess to make room for fresh signings, as the new coach's arrival has requested.
It's not a rumor, Mason Mount has stated publicly that he will not sign a contract extension and that he intends to leave the team. Despite knowing that he is worth the money, Manchester United is unwilling to pay the price tag on his head because he has expressed interest in signing with them. Arsenal will take Mount if Manchester United doesn't close this deal soon.
when every season ends this is an opportunity for every team to make overhauls to each player and including Chelsea who have had a new coach whose aim is to revive his strong performance after a bad season for Chelsea at that time. so it's not surprising that many players will be replaced with new ones because Chelsea may need players who really have talent and good contributions to the team.
maybe Mason Mount decided not to renew his contract with Chelsea for several reasons but this is just my speculation that Mason Mount prefers to leave Chelsea because of a new coach who is unlikely to be able to make Mason Mount comfortable with the strategy given because it takes a long time to can get to know the new coach better so decided to leave Chelsea and I think Man United really want it and it seems like they are trying but I don't see Arsenal's interest in taking Mason Mount.