This is exactly what we need; a defence leader, a thing which Milito can provide to our back line. In my opinion, a partnership between Milito and either of Grygera, Zebina or Criscito would work. As for Barzagli, I don't think he'll add a lot to our back line after bearng in mind the ridiculous amount of money Zamparini wants for letting him go.
No, Barzagli isn't underrated here at all, on the contrary I think he is just a media hype. And when Lippi chose him amongst his team to the world cup, he was the 4th choice after Nesta, Cannavaro and Materazzi. In my opinion, the main cause that forced Lippi to chose Barzagli was the nonavailability of classy centre backs, hence it was Barzagli.
And I don't see why the presence of just Milito as our lone newcomer defender wouldn't work. We still have good defenders to partner him. Grygera is a good sane right back who can play in the centre as well. I remember Thuram was a right back at Parma, when we switched him to the centre, despite his old age and his lack of playing time in the centre in his first three seasons at Delle Alpi, he adapted quickly and he did a good job. Criscito is considered the best centre back in his generation, I'm not with fielding him as a regular starter though. But he can settle and have a fair playing time which will make him mature more quickly. But with the presence of lots of centre backs, I can see us loaning him out soon. Zebina, despite being clumsy and a hothead sometimes, is still a solid defender. With the presence of Chiellini, Milito and Grygera alongside Zebina, I'm confident that we'll have a solid competent back line.