I really think Boumsong is gone before August 31st, and Legro will stay as a backup. As well, I fully expect one more defender of moderate to excellent quality who can rotate with Criscito. If Ranieri wants Chiellini to be that CB, I'd actually go for Zaccardo (yeah, I know... hear me out)... he's a guy who had a lot of potential, but due to some very notable mistakes, his career is plateauing. Even so, he's way better than Molinaro, Legrottaglie and Boumsong... and in a new setting with new responsibilities, he might be able to regain a spot with the Azzurri. Palermo is definitely more willing to deal him than Barzagli, despite the new contract signings, and would be a lot cheaper. He's in the right age group (25-28), has proven Serie A experience, and would likely accept the fact that he might not always start unless he's consistent and superior to his competition.
If Ranieri wants to keep Chiellini at LB... I'd go for Cannavaro as a 2-3 year stop-gap while Criscito matures.