The former Juventus star has been an impressive figure for the Premier League club in recent seasons.
Postecoglou admits that not only has Kulusevski shown more on the pitch, but he has also improved as a leader.
He told reporters: “I think Deki is growing but I think that's kind of where we are as a group. A lot of them are growing and growing through experiences. I think for Deki at the weekend was another growth period because he got hurt pretty early and wasn't happy with it. In many respects, he lost a bit of his composure after that and that's another growth for him.
“To understand it is part of it and focus your energy on helping the team on the day rather than being personally aggrieved, but that's where we are as a group and that's where we get our growth. By going through experiences like we did at the weekend with a lot of the players and them going through that and understanding how to deal with it better next time.
“Deki has been fantastic for us this year in both his output and his attitude and mentality. I think there is more to come.”