I don't know of a phrase where someone can so generically point out a flaw. In your example, I'd say human - nobody is perfect. Perhaps his weakness or limitation would fit here.
There are words for dramatic flaws.
An Achilles heel: a portion, spot, area, or the like, that is especially or solely vulnerable. It does not need to lead to his downfall; it connotes someone who is pretty damned perfect except for this one thing. "Tom Cruise has an Achilles heel" would be understood that way.
His Achilles heel is his quick temper.
Shortcoming, or foible would be understood as an understatement; if he's crazy, that's a major problem; to call it a foible before stating the flaw would be mildly sarcastic.
A tragic flaw in drama is a failing of character in the hero (someone of high repute/honor/etc.) of a tragedy that brings about his downfall. Regarding Hamlet:
He has exceptional qualities like graceful personality and popularity among his country that is eminent. His tragic flaw is his delay in action and irresolution...
Other expressions for this are simply flawed, or inadequacy.
Best Answer
There are lots of terms: sports fan, sports fanatic, sports aficionado, sports enthusiast, sports spectator. None of those, however, indicate that he watches only sports and nothing else. There isn't a term for that. If you want to say something to that effect, you'd say something like, "He's faithfully married to sports spectating, never having a wandering eye."