To be fair he was a quality player in our 2014-5 season. Just a classic example of not knowing how well you had it until you left. Perhaps if he had stayed at Juve he would have developed into a good player.
I think he's mentioned psychological issues in the past. Wouldn't surprise me if he comes out one day and talks about suffering with his mental health, he's very streaky in his form but also always was a better reactive striker than a clinical one.
Unfortunately I think any player with these characteristics is going to struggle to play at the really top level in any professional sport. As it is his speed and directness has carved out a career for him. By the time his legs have gone he could end up like Fernando Torres at the end of his career.