I think his issue is arrogance rather than partying. He's from a very well-educated family and I think he brings a noticeable degree of arrogance with him. Manziel seems like the wrong negative comparison. I'd say the better comparison is Jay Cutler/Aaron Rodgers.
UCLA is a 2-9 team with a regular qb, and they'll get to .500 with him.
PS - with regard to Rosen, something tells me that he's not a McDermott/Beane type of guy. If that's true, I think that's a shame because he is very good and you have to be able to put up with some difficult players if you're going to maximize talent on the team.