Captain Michael Hooper was missing from the Australia squad named on Thursday for two home Rugby Championship Tests against South Africa as veteran playmaker Bernard Foley earned a shock recall.
Hooper pulled out of the team's two-match swing of Argentina this month a day before their opening clash and jetted home, saying he was not in the "right mindset" to lead his country.
In his absence, the Wallabies won the first Test 41-26 but were humiliated 48-17 in the second.
Rugby Australia boss Andy Marinos previously said "you cannot put time limits on these things" when asked about Hooper's return.
Coach Dave Rennie said he had spoken to the 121-Test veteran since their return from Argentina.
"We're not in a position to discuss that in detail and we've got no firm timeline as to when he will return," he added.
Fellow Test centurion James Slipper, who took over as skipper in Argentina, is again expected to captain the side for their showdowns against the Springboks in Adelaide on 27 August and Sydney a week later.
Japan-based Foley, 32, returns for the first time since the 2019 World Cup.
The 71-Test fly-half, who plays for Kubota Spears in Japan, gets another chance with Quade Cooper out with a season-ending injury and James O'Connor dropped after starting Australia's last Test.
"Obviously with Quade's injury and his (Foley's) availability, we started having a conversation after the first Argentinian Test," Rennie said.
"James (O'Connor) got a crack in the last Test and we wanted him to drive the ship and allow us to implement our plan, but we were clunky and lacked cohesion, so he's missed out on selection.
"Bernard hasn't played an enormous amount of rugby recently... we'll make a call on him once we get him in camp on Sunday."
While Hooper's absence is a blow, the Wallabies welcome back a host of players from injury, including winger Andrew Kellaway and props Angus Bell and Scott Sio.
Fellow prop Allan Alaalatoa, centre Hunter Paisami and hooker David Porecki also return after leaving the Argentina tour early for personal and injury reasons.
Young Waratahs loose forward Langi Gleeson was named in his first national squad, with Reds fullback Jock Campbell the only other uncapped player.
"To have their experience on deck will be invaluable in what's going to be a massive challenge coming up against South Africa two weeks in a row," said Rennie.
"We know that respect is earned daily and we'll be doing everything we can to ensure we put in performances that are worthy of that respect in Adelaide and Sydney."
South Africa head into the Tests on the back of a 35-23 home loss to New Zealand, having stunned the All Blacks 26-10 in their Rugby Championship opener.
All four teams in the southern hemisphere competition have won one and lost one so far.
Australia squad: Allan Alaalatoa, Rory Arnold, Angus Bell, Jock Campbell, Pone Fa'amausili, Folau Fainga'a, Lalakai Foketi, Bernard Foley, Nick Frost, Langi Gleeson, Jake Gordon, Reece Hodge, Jed Holloway, Len Ikitau, Andrew Kellaway, Marika Koroibete, Rob Leota, Noah Lolesio, Lachlan Lonergan, Tate McDermott, Fraser McReight, Hunter Paisami, Jordan Petaia, Matt Philip, David Porecki, Pete Samu, Scott Sio, James Slipper, Darcy Swain, Taniela Tupou, Rob Valetini, Suliasi Vunivalu, Nic White, Harry Wilson, Tom Wright