Australia's Sean McMahon is tackled by South Africa's Francois Louw (top) and teammate Pieter-Steph du Toit during their Rugby Championship clash.
Australia's Sean McMahon is tackled by South Africa's Francois Louw (top) and teammate Pieter-Steph du Toit during their Rugby Championship clash. Themba Hadebe

Wallabies suffer another back-row blow in loss to 'Boks

RUGBY: Wallabies coach Michael Cheika's back-row tinkering looks set to continue following Sean McMahon's ankle injury in the 18-10 defeat to South Africa in Pretoria.

McMahon is set for scans this week to find out the extent of the damage but Cheika said it was likely the 22-year-old had sustained a "pretty significant” injury.

The Melbourne Rebels forward played No.8 against the Springboks in place of David Pocock, who missed the Rugby Championship clash with a broken hand.

McMahon was expected to get a chance to make the position his own but now could face time on the sidelines and miss at least this week's game against Argentina at Twickenham in London.

The No.8 had the injury strapped up but was unable to make it back on the field, with Cheika adding: "That's two now in that spot.

"We'll see where we end up next week.

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

With McMahon possibly missing the game in London, Lopeti Timani, the only other recognised No.8 in the squad, could face the Pumas.

Timani did not make the bench at Loftus Versfeld, having made his debut as a reserve against Argentina in the previous game in Perth.

There is also the possibility Ben McCalman could earn a recall after recovering from a shoulder injury.

Wallabies vice-captain Michael Hooper was still hopeful McMahon would not miss a match.

"He seemed pretty positive,” the back-rower 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.”

The defeat meant the Wallabies are still to break their duck in Pretoria.

They have now lost all seven games at the high-altitude venue.

Fly-half Morne Steyne celebrated his recall to the team, kicking all of the Springboks' points.

The victory ended their three-match losing streak in the competition, which has already been won by New Zealand.

Steyn, starting in place of struggling Elton Jantjies, kicked four penalties and two drop goals as South Africa moved up to second ahead of its last game of the series against the All Blacks.

Scott Sio dived over under the posts after taking a pass from fellow prop Sekope Kepu to score the only try of the game.

But the Wallabies squandered further chances despite dominating possession in the second half.

Meanwhile, the All Blacks stayed on track to break their own record of 17 straight victories with a 36-17 win over Argentina in Buenos Aires.

The New Zealanders ran in four quick tries in the latter stages of the first half to set up the win.

Steve Hansen's side has now won all five of its Rugby Championship fixtures and will look for a clean sweep with a win over the Springboks in Durban next week.

The All Blacks will claim a record 18 consecutive Test victories if it wins on the weekend and in Auckland in the third Bledisloe Cup clash.