I seriously don't see Buffalo drafting Rivers at #13. The last two first round picks they used on a guy who couldn't help them in his rookie year, and a QB. I don't think they use another first round pick on a guy that can't help them immediately. They REALLY need some WR and passrush help.
There's a couple of guys like Tommie Harris, Kenechi Udeze, and Will Smith that they would have to look at on defensive line, then there's the WR position and they'd have a pretty good shot at Reggie Williams where they are.