Personally, I think Rivers' game would benefit from another year in college.
If he really is going to enter the draft, he could be anywhere from a top 10 pick to mid-late 1st rounder right now. On talent alone he's top 10, but will there be a team that needs a SG and thinks picking him in the top 10 is good value?
As a far as the Celtics potentially picking him, I hope they do not. Just seems like far too risky a situation. I don't like that father/son coach/player dynamic at all. While it could work really well, if things go bad, it would have the potential to be really bad.