His injuries ruined him.
He looked afraid as soon as he joined Chelsea, complete shot of confidence. Instead of that huge fee making him feel like a complete world star, it hindered him and his average performance was as a result of him taking less risks;Ie less shots, less of them bursting runs, more hash toe punts etc.
Always trying to get the ball away asap, which shows you some glimpse into his mentality. Similar to Ozil, top top class as far as talent goes, but football is a mixture of physical as well as mental, neither are great mentally.