There are definitely concerns about him, but I think that's the thing that might be key. I've read a lot of people talking about his leadership and intelligent play - and that's more important at City where the defence is so exposed that a single mistake, or player out of position can cause havoc.
Stones has all the attributes of a great defender, but would definitely benefit from a strong leader beside him. Garcia looks good, and it sounds like he's going to be a leader/thinker on the pitch, but I just can't believe he'll prosper in the PL with his lack of height and pace.