Ray Allen's short term replacement is probably already on the roster. Avery Bradley. He will have to prove he can be the long term solution though and that will require him proving he isn't two small to play the 2 full-time.
Ray Allen is not a dominating physical presence (6'5" and maybe 200 lbs) by any stretch, and he's been playing SG in the league for 16 years.
AB might be only 6'2", 180 lbs (my guess is he gets to 185-190) but he has the wingspan of someone who is 6'7"
AB has speed/jumping in spades over Ray, including the young Ray. AB can challenge any jumpshot by a 2, save for a Kobe-like scorer who is 6'8" with range and a fadeaway. But no one can defend those guys well, certainly not Ray.
And AB will be one of the best on-ball defenders in the league, if not the best. I actually think he will defend the opposing PG more while Rondo plays on the 2/3 and sags into passing lanes.