Adam Johnson. Mancini used to get pelters from sections of the media and City fans for not starting him enough, playing him on the right and "ruining a young English talent". Did he join a top 6 team after leaving and go on to become a fixture for England? No, he went to Sunderland, and the rest, as they say, is history...
Balotelli. I liked the guy, but mavericks and "characters" tend to be overly romanticised and their ability can get over exaggerated a little. "If only he applied himself, he could have been..." I don't feel Balotelli wasted his talent from not applying himself. He has a great CV. He also played in a Euro's final, so you can't really call him a wasted talent. Just my opinion, but I feel he was as good as he could have been really, I don't feel he had a higher ceiling that he failed to reach.
Not overrated as such, but Zinchenko took on an almost mythical status after that Villa game. He went from being a squad member, to becoming an indispensable player almost on the same level as the likes of De Bruyne and Kompany.