Tricky one to answer. He had a **** poor first season. (And was equally as bad the opener this Sunday.).
But he is still just 21 and developing. And the ever changing back line, all to no real quality, around him at Milan hasn't helped. He's shown flashes that he will be a composed ball playing centre half. Very comfortable in possession and a good carrier/ passer of the ball out of defence. Big lack of pace. He'll be in the classical Italian mould you feel of being positionaly savvy to compensate.
Like Patrick said, I don't think there's any chance of Milan cashing in unless a stupid offer tipped up. And honestly, if I'm Chelsea, I wouldn't be looking at him. Not ready right now to consistently perform the PL as he grows. And the complete wrong type of centre back for a league where speed is a pre-requisite if you're wanting to build.