The Romania defender has been linked with a return to Italy in January, where Juventus are keen.
But Postecoglou said on Friday: "Radu's been with us 10 months now and I think he's been great. We signed him because we saw some really strong attributes in him in terms of his defending capabilities, his mindset and again a really unique journey that he had to fight hard to get where he wants to. So, he had all those things I look for.
"He is still young and he is working hard every day on his game and all aspects of his game. Defenders you know they tend to get better with age with experience, the good ones, and with him, guys like Micky (van de Ven) and Destiny (Udogie), the reason we signed them at the age they did is we really believe in years to come they'll get stronger and he is doing that.
"Every time he is out there I think I can see improvements. He is really embracing the way we play. It is very different to where he came from, playing so aggressively with our line and defending one-on-one but he's embracing that and enjoying that.
"I thought he was great the other night. Not just in his football but in terms of his presence and demeanour, he took some leadership there. Obviously, we lost two of our back four during the game with Micky going down and then Cuti (Cristian Romero) out. He had Archie (Gray) next to him who is 18 and I thought he really stood up in that time just to make sure with those disruptions, we maintained our organisation and I thought he was really good."