It's simple. We don't have a LB in the squad. Peluso is a LB. And he can play as a CB, a position where we really need depth.
From the little I've seen, he looks like a decent player. Tall, fast, decent passing.
One thing regarding this that bothers me though is that clearly nobody trusts De Ceglie, maybe not even to be a backup. He could very well remain in this squad to fill in the homegrown quota for CL, along with Marchisio, Gio, Marrone.