I think our pressing from the front has been much worse this season which have affected our defensive performances. Of course the probably relaxed a bit, at the same time they were very lucky in their first 32 games conceding way less goals than they should have, and not thanks to Alison saving them but thanks to the opponents missing the target.
I still rate Stones and think that if he can stay injury free he'll be good enough for us. Of course if his form is that same that its been for the last 1,5 years then Koulibaly will (hopefully) prove to be a upgrade but I think Stones will come good. And while Koulibaly obviously is a good centre back I'm far from convinced he's one of the best in the world (are there any real good centre backs out there apart from vvd?) and might look even worse in our system.