Postecoglou is convinced Spence is performing as well as any fullback in the country.
"Fair to say in terms of full-backs in recent times, his form is as good as anybody's," the Australian said. "I think he'd be up there.
"But for him, the challenge now is, ‘Don’t settle for that now. Push on’. There’s no reason why he can’t. He can be a top, top player. He’s already at a great level but don’t settle for that. My role and for all of us here is to keep pushing him on."
Postecoglou was asked if Spence could handle an England call.
"He’s got a really laid-back personality...and sometimes that gives a false impression.
"That laid-back nature also means he won’t be fazed by anything that will be thrown at him. It works both ways. What some people see as maybe a weakness in him, that he looks a bit too casual, but...come game-day he’s not fazed by any challenges thrown to him. I think that’s partly why he’s done really well since he’s been thrown in.
"He’s had some difficult experiences but that’s what life is about. Those difficult experiences can actually help you. He’s played in Italy, in France, in the Championship. I’m sure he’s taken those things along, even if they haven’t been positive, and said, ‘OK, there’s nothing really to fear, here’. I think he takes that into games."