First-choice right-back Franjic limped off early in the second half of the pre-FIFA World Cup match in Sydney after sustaining a knock to his knee.
The Brisbane Roar man is expected to be able to travel to Brazil with the rest of the Australia squad, but Postecoglou is likely to be sweating on the fitness of one of his key players with just one friendly remaining before their World Cup opener against Chile on June 13.
Postecoglou had captain Tim Cahill to thank for ensuring Australia did not sign off their home-based preparations with a defeat, as he popped up with a trademark header to equalise just one minute after Ayanda Patosi's early opener for South Africa.
A frantic opening to the game brought two goals inside the first 15 minutes.
First, Patosi finished smartly past Mat Ryan after a good move down the visitors' left-hand side.
South Africa's lead lasted just 60 seconds, though, as Cahill levelled matters from the hosts' first attack after the restart.
New York Red Bulls' midfielder Cahill, playing in a more advanced role for his country, could have put Australia ahead a minute later, only to be foiled by a superb save from Senzo Meyiwa.
The game then settled into a pattern of end-to-end attacking football, with Australia threatening on several occasions.
Substitute Ben Halloran went close with a long-range effort, before Dario Vidosic missed a great chance to snatch a winner late on.
Postecoglou was forced to settle for a draw, but his primary concern will be the fitness of Franjic ahead of a daunting World Cup campaign.
