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Cape Town - Australia's loss to South Africa in Pretoria proved costly with the news that they are sweating on the fitness of back-rower Sean McMahon.
McMahon hurt his ankle during the first half of his side's 18-10 defeat and has been sent for scans to determine the severity of the injury.
The 22-year-old tried to play on after having his ankle strapped but battled to run and was replaced at half-time by Scott Fardy.
Wallabies head coach Michael Cheika said it looked like a serious injury.
"That's two now in that spot," he told AAP.
"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 number eight in the current Australian squad, against Argentina at Twickenham.
Alternatively, Cheika could fly over Ben McCalman, who has recovered from a shoulder problem and is back in action in Australia's National Rugby Championship.
Wallabies vice-captain Michael Hooper is still hopeful McMahon won't be ruled out.
"He seemed pretty positive," said Hooper.
"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."
