Imo we don't necesserily two new top-defenders for next season.
We have Chiellini,who I would play as CB and Criscito and Boumsong as back-ups.
Et voila':
Grygera---Milito---Chiellini---Criscito
If it doesn't work, Zebina could take Grygera's place,the same with Criscito,Molinaro could play instead of him.
Milito is the leader, but don't forget Buffon and even Chiellini,who is U21 captain of Italy,they have leader-abilities too imo.
Backups:
Rb Zebina
CB Boumsong
CB Zebina,Grygera
LB Molinaro
Maybe we need another back-up CB?