Fact is Tevez had what will be the worst season of his career(while also winning the title and being insanely crucial to winning the title, odd isn't it
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) in terms of training, not being a part of a team. He spent several months barely playing and a few months barely training, not playing reserve or training games or possibly football of really any kind and still looked fitter, though larger, than Rooney after preseason.
Difference is there was a huge reason for Tevez to get out of shape, and when he was back he was commended by loads of people for training insanely hard to get fit. What did he, a professional athlete/sportsman do over summer, put more weight on, come back and half arse preseason? No, he dropped weight and looks fitter than ever and has been instrumental in all their games so far.
What did Rooney do, he put on weight, clearly didn't get particularly fit in preseason while the rest of the footballing world did. Frankly he just comes across as fairly unprofessional, as does his general behaviour/attitude on the pitch.
At 18 you can get away with some light smoking, some big party nights, going to all the press/launch/celeb events, letting it all go to hell in summer and coming back and working insanely hard to get fit.
I doubt you can name a single athlete still going strong at 35+ who has that attitude. You have your Ricky Hatton's who half arse their entire career, get shown up against the best, let themselves go between fights and fight an unhill battle against their own body because of their own stupidity, and someone like Mayweather who stays very close to his peak between fights, who works hard, and is still ridiculously fit at a late stage in his career.
Maybe the real question is, does he have the desire or motivation to step up, get fit, stay fit, lean, strong and focus completely on his fitness rather than other things. Thing is he's rich, he's on a huge contract for a few years more at least anyway, does he have a whole lot of reason to try that hard, will he at 26 turn into a model pro to lengthen his career or will he be happy with what he has and keep going backwards. Two years in a row(or is it three?) his summer/preseason has been a complete joke, he's come back not even slightly prepared for either season, having to go away to America to get fit when the rest of the team/league get it right is rather, embarrassing. You'd think after the previous issues with fitness he would, if he was going to get it together... not have had the issue again this season.