I would argue his value has in fact risen with the amount of games he has played for us this season.
I think every player has had a worse season than the previous season
You are also completely ignoring the current situation
also how many games did he play this season vs last ? Mendy was injured for both his injury started last hence why we bought Angelino Angelino has not played many but he played a few we did not have him last season
in any event no way has he gone from a 15 million player to a 25 million player