Postecoglou admits the Argentina defender is one of the major leaders at Spurs.
He said, "Outstanding. Obviously with all these kind of things you have the image of a player from afar and before I got here, I thought he was going to be really important to me because he's such a fantastic centre-back for club and country.
"But being here, watching him, the way he trains and the way he plays, unbelievably strong mindset, an absolute winner in every thing he does.
"He was really important last year and he's going to be really important moving forward. He is one of the players within this group who has achieved things from a team perspective and those kind of things are transferable. He has won the World Cup, he has won the Copa America, so he's part of a successful team and I think he's a really strong influence in the group."