Would rather a Barzagli replacement than a Chiellini replacement.
I think the club see Caceres as that, it's just that Barzagli was rarely injured or showing fatigue.
Ogbonna can play similar positions to Chiellini but he's a bit different in terms of style. He has the physique to play tough but is not as determined a defender. He is better with the ball at his feet than Chiellini, however.
If Peluso is going to be retained it does make us look pretty stacked in terms of back three or back four options. From what I've seen of Ogbonna he is quite comfortable at left back, more so than Chiellini (who I don't think is a bad left back).
The issue of the vice-Bonucci would remain, if we stay with the three and let Marrone go out.
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I think he is over rated. He is not even chosen as a backup for Italy NT.
Probably more to do with Astori being the more natural replacement for Barzagli, with potential injury issues. De Sciglio is the new bright young thing at fullback and looks decent, so it's hard to take his place at left back. The only other way that he could get in is to drop one of the two natural right backs.