I don't think he's finished at all. If Stones could come back why not him then?
He has to follow his example and work hard. He is the victim of Pep new system and also of Cancelo explosion
I'm gonna try to explain myself. Cancelo is out most important fullback for many reasons and he is the first name Pep puts on the game sheet. If Cancelo is on the right , Pep will play Zinchenko on the left to have a defensive balance( Zinnie doesn't play as high as Mendy, actually Zinnie plays Left back and midfielder, Mendy plays left back and winger)
If Cancelo plays on the left , Pep will play Walker on the right for the same reasons. If there was no Cancelo, Pep would be happy to use Mendy to have this offensive threat.
The challenge for Mendy now is to learn how to play as a midfielder and left back in the same time. With work he can do it and if he can combine the 3 fonctions in the same time like Cancelo(left back, winger and midfielder) then he will have his chance.