The problem last night is he took too many risks. He was running with the ball, and making silly mistakes. Now this is probably because he was out of position.
What I rate most about him is how cool headed he is when he is on the ball. When he's at CB i don't think he is in the position to be bombing forward and taking risks. He would be an improvement on Mangala currently, but only slightly.
If City are thinking about a bid for him, we need to think who will develop into the better player, Stones or Mangala? If it's not an obvious decision, then don't spend the money on him.