That's a fine line between exhaustion and injury caused by it and strengthening. You need to take time off and come back slowly, pushing too hard can re-aggravate injury. That being said, there is no 100% answer to his issues. It's partially mental, it's partially physiological, it's partially external and internal pressure.... it's a bunch of stuff.
Generally, professional athletes aren't really "lazy" as we would be. They are professional athletes, after all.
However, anyone remember Mario Goetze? Scored a WC winning goal, and not injury, not anything happened to him, he was just gone, never the same.