The Wallabies were dealt a blow when it was confirmed that back-row Sean McMahon has been ruled out of next weekend's Rugby Championship clash with Argentina.

McMahon was sent home after suffering an ankle injury in Australia's 18-10 defeat to South Africa at Loftus Versfeld in Pretoria on Saturday.

The 22-year-old had scans after the Test and the early diagnosis is syndesmosis, or a high ankle sprain, which could sideline him for up to a month.

"They can vary in degree," Wallabies defence coach Nathan Grey told AAP.

"I think he'll get home and get confirmation of what the depth and length of time he'll be out.

"It's disappointing for Seany after a really good input to the game."

The injury will send Wallabies head Michael Cheika back to the drawing board for another solution at number eight, with McMahon having been brought in to replace David Pocock who is sidelined with abroken hand.

Test newcomer Lopeti Timani is the only recognised cover for that position in the current squad and he could make his starting debut in the clash with Los Pumas at Twickenham.

While no player will be flown over from Australia to boost the squad, Leroy Houston – who is playing on loan from the Queensland Reds for English Premiership side Bath – looks a good chance to be called upon.

Houson, 29, was part of the Wallabies' training camp prior to the three-Test series against England in June, having been coaxed to leave Bath and return to Super Rugby and the Reds by Cheika with the carrot of potentially representing Australia.

"Leroy, obviously, has been in the program and sort of knows how we play," added Grey.

"It's a good option for us to have up our sleeve. He'll be match fit as well so it's a positive."

Prop Scott Sio, who scored his first Test try in the opening half in Pretoria, said McMahon's presence around the squad would be sorely missed.

"He brings a lot of energy to the team," said Sio.

"I call him the Bulldozer, he's a friend of mine – that's just the way he plays. He only knows one way. He's quite a character in the team and that's what we're about here, putting it together on and off the field."