The Wallabies are sweating on the fitness of Sean McMahon as coach Michael Cheika faces yet another back-row configuration for next weekend’s Test against Argentina.

McMahon has been sent for scans on his ankle after injuring himself in a tackle in the first half of Saturday’s 18-10 Rugby Championship defeat to South Africa (Sunday morning AEST).

He tried to play on after having it strapped but struggled to run and was swapped at the break for Scott Fardy.

The 22-year-old was brought into the team as a replacement for David Pocock (broken hand) and was set for an extended run at No.8, a position he is keen to make his own.

Cheika said it was likely a “pretty significant” injury

“That’s two now in that spot,” Cheika said.

“We’ll see where we end up next week.

“He was going well when he was on there, that was very positive.”

McMahon’s injury could mean a starting debut for Lopeti Timani, the only other recognised No.8 in the current Australian squad, against Los Pumas at Twickenham.

Timani was the last man trimmed from the Wallaby bench at Loftus Versfeld, having made his first international appearance as a reserve in their previous Rugby Championship clash in Perth.

Alternatively, Cheika could fly over Ben McCalman, who has recovered from a shoulder issue and is playing back home in the National Rugby Championship.

Vice-captain Michael Hooper is still hopeful McMahon won’t miss a game, describing him as a player they “need” in the team.

“He seemed pretty positive,” Hooper said.

“He’s a hard, hard bloke, so it’s going to take a lot for him to not play.

“We’re hoping for the best there because the involvements he had early on in the game were fantastic – a couple good carries, turnovers, tackles.

“We need him in the squad and hopefully that’s the result we get.”