I think 'attitude problem' implies that he's deliberately being difficult or something, when in actual fact he doesn't strike me as being difficult or doing anything negative. Perhaps he's just struggling with the weight of expectation; from Middlesborugh to Aston Villa, Champions League, England and being YPotY is quite a meteoric rise.
People are calling him out for offering nothing, to be dropped, for having poor attitude etc. I have no doubt he's trying his best; perhaps though he's just not as relaxed as he was. Lord knows how most of us would have performed at 23 with that weight of expectation and scrutiny.