City bought into the hype around him and probably watched a few videos of his ball playing skills and thought "Man this guy could be our own Pique!" without the defensive talent.
When I look at Marlon,Umtiti and you compare them to Stones it's interesting. Marlon is only a year younger than Stones, and despite the lack of top level European experience you can see he has the foundations to be a really good solid defender.
Umtiti is only a year older than Stones but the difference between them is quite stark to say at least and by watching both you can easily tell who's the better player and it's not even a contest at this point in time and possibly even ever.
With Stones well to be he seems to be lacking the proper defensive foundation he should have at this age at least. Is it down to him? Martinez? is it Pep? or all three perhaps? at this point in time Stones looks lost at what to do.
Time will tell with Stones obviously since he's only 22 years old but it's worrying for him that theirs a lot of players around his age group who are so far ahead in terms of development than he is.