Even if you set aside the attitude and assume the new boss can overcome that, he's never been particularly gifted technically, instead relying on pace. As others point out, that appears to have faded in recent times - perhaps because of injury, which can happen. The fact he hasn't really adapted his game to develop in other areas to compensate for that has been disappointing.
He's quite a tall player, he could/should have developed into a player with intimidating physical presence in the box, but even in that area he's wilted rather than developed, now seemingly allergic to most forms of physical interaction on the field. He would have benefited maybe not having the 30 goal a season period, the slump coming earlier in his development and that acting as a motivation to adapt his game.
I don't think physically he can do what he did two or three years ago, but the problem is now he can't really be anything else. For the occasional match, maybe even very seldomly a short burst of games, it might work. But for the long-term someone should have been pushing down the pathway of adapting his game.
Football is full of players who look so promising aged 22 with pace to spare who by the time they're 28 look clapped out. I wonder why more aren't coached on basis that their speed is often only a fleeting asset not something to build their whole game around to the exclusion of everything else.
