I can't help but feel this team has two glaring weaknesses in its starting 11...
Molinaro and Nocerino
Nocerino may very well be displaced by Sissoko, so that covers one, so long as Sissoko himself doesn't become a weak link
That leaves Molinaro. With Ranieri unwilling to play Stendardo and move Chiellini to LB, that means something has to give before CL teams rip that left side to shreds. Grosso and De Ceglie would each be an upgrade to Molinaro, and the former would be a good mentor for the latter.
After that, the major thing to look for in mercato is just a general upgrade bearing in mind that the heavy lifters in forward and midfield will all be 30+.