I honestly think if he wasn't a petulant man baby he could end up being one of the best in his role in world football. He's got some fantastic natural ability - low centre of gravity, he can glide past players, his ball manipulation is genuinely excellent too. His passing is really good too, numerous times under Mourinho he broke through the lines with a carefully picked pass.
Not currently one of the best though, or even close to it. But all the tools were there imo. If he worked hard at his fitness on the training ground every day, fine tuned his game, listened to the coaches .. he could have been incredible.