The value/importance in a CB for Man City is in their positive traits and not in their defensive traits. Stones has things you can't teach in the positive side and he's not terrible defensively. He'll be fine. Nico/Laporte will anchor the defense and he can work on his game during this downswing.
Edit: I think it's a little bit silly for people to get wound up over occasional blunders from our defense. Relax. We're offensively orientated + a few blunders are inevitable. It's actually amazing at how little we concede. Klopp is outstanding at what we're doing (all offense) but he can't even come close to us defensively. I'm backing his style to finish top 3 next season and we're so much better at it than him.