I think once players start stringing games together then the mental aspect and doubts will hopefully disappear.
Who knows though, strikers who continually struggle with hamstring injuries (Owen and Saha spring to mind) mentioned always having to "manage" how they played so I guess it depends on the injury.
It's looking increasingly likely that he had another injury at some point because Ancelotti was definitely discussing him in November and that he was close to being back with the first team training. That was over 3 months and still no sign of him being with the 1st team.