Captain Mile Jedinak (groin), defender Curtis Good (hip flexor), attacking midfielder Tom Rogic (groin) and goalkeeper Mitchell Langerak (knee) are all struggling for fitness ahead of Australia's final friendly before they head to Brazil.
And Milicic declared only players who are 100 per cent fit will take on South Africa in Sydney, while he explained Postecoglou would most likely use an experimental line-up at ANZ Stadium as he attempts to finalise his squad for the FIFA World Cup.
"He (Postecoglou) doesn't need to risk anybody who's got any question marks (over their fitness)," Milicic said on Saturday.
"It's not a game where we have to risk any players and there's probably players that he wants to see a little bit more than others and he'll make that call."
The 40-year-old assistant coach added: "You're giving someone else the chance, you're bringing in younger players who will, I'm sure, want to impress the selectors, (they'll be) young and keen.
"For me, it doesn't matter where you come from. If you get called up for your national team, not one player doesn't give 100 per cent. It will be a tough game.
"The main thing for us to get out of the game what we want in terms of our style we've been working on in the last week."
Australia have been in camp in Gosford for the past week, as Postecoglou, who only took over as coach in October, continues his rebuilding programme.
Although South Africa's squad has been ravaged by injuries and withdrawals, Australia defender Ryan McGowan warned the visitors that Postecoglou's men will not be complacent.
"(Postecoglou) will want to get some rhythm to the game...he will want to see a few of the younger players I guess but at the same time he doesn't even like losing training sessions so I can't imagine if we have a bad performance he'll be too happy," McGowan said.
"He'll definitely want to put on a show for all the fans that come out and we'll want to make sure we leave Australia on a high."
Postecoglou's men will start their World Cup campaign on June 13 against Chile before taking on the Netherlands and Spain in Group B.
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