The thing is his ceiling is pretty fucking good. It's his whole body of work put together that's pretty bleh. He came in for the Bears a couple years ago and threw for a ridiculous amount of yards and one pick over the second half of the season. That actually happened. It can't be denied and you can't take that away from the guy. So at a minimum that's his ceiling. And it's higher than anything Johnny Manziel will ever reach in my opinion because that guy is terrible. And not only is he terrible, he's a headcase. I wouldn't want him in my locker room, much less the leader of my team.
Er...McCown is 36 years old. He's in his 14th year in the NFL. I would think that the half season in Chicago is an outlier, since he's been bad to average everywhere else.