It's been suggested the Italy left-back could attract an approach from Manchester City this summer.
In response, Postecoglou said: "I don't know about the speculation, but he's outstanding.
"From the moment I arrived I felt he's going to be a real outstanding player for us. So much growth still to come, physically he's got outstanding attributes for that position, but he's improving his game all the time.
"He's obviously had two seasons where he's had some injury issues that have disrupted him a little bit, but even that, I just think it's part of his growth physically and coping with the levels.
"He's hugely important to us. His profile as a left-back, in particular the way we play, but in general is pretty unique in that he's big, he's powerful, technically he's good going forward but also a very good defender.
"He's in a good space at the moment and I still think him, with a number of others, I just think in the next two to three years they're going to be really, really important players for this club."