Son has complained of fatigue this week after being taken off during victory over Europa League opponents Qarabag.
Postecoglou said: "It's something that I'm mindful of but it's just the circumstances so far, and Sonny always wants to play, that’s his attitude, but we’ve got to be sensible about it. I don’t think it’s got so much to do with his age because I haven’t seen that affect him.
"It’s more I just don’t think that kind of workload in the modern game is sustainable. We’ve spoken a lot about fixture overload and part of that responsibility lies with us to try to protect our players and certainly with Sonny we’re going to have to be mindful of that."
On whether he'd ask Son to lessen his commitments to South Korea, the manager also said: "Oh no chance, because I understand how important it is for him. We can look in the cold harsh light that his club football will benefit if he doesn’t play for his country but one day he won’t be able to play for his country and the one thing I wouldn't want is for him to have regrets that he missed the opportunity to play as much as he could.
"All these decisions are better left to the player because they know how they feel about their career. It doesn’t matter how long you play, the lifespan of a professional footballer is still fairly short compared to how long you live your life so the time you have you want to be as fulfilled as you possibly can.
"You want to create as much of a basket of memories as you can because one day you won’t have that opportunity. I would never be the one to say, ‘Look for selfish reasons, for us as a club it would be great if you didn’t play for you country’. I just wouldn’t do it."