The midfielder once again played an impressive role in a 2-2 draw against AS Roma in the Europa League group stage.
Gray has been used more sparingly in the Premier League, but has been getting ample game time since moving from Leeds United in the summer.
Speaking about his maturity, Postecoglou told reporters: “There's a real maturity at only 18. We asked him to play left-back, he has never played that position in his life but does it with a real efficiency and calmness.
“I am so excited by Archie and that's another positive for us that we have another 18-year-old who just plays and no one mentions that he is 18, they just measure his performance and, like I said, I am asking him to do different jobs. It would probably be more comfortable for him if he was settled in one position but he is playing football at a really high level in Europe and the Premier League.
“That's going to set us up really well for the player that he is going to become. I thought Lucas (Bergvall) was really good when he came on last night, he made an impact. With everything else we're building, there is some real excitement around the young players as well.”
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