James Horwill has a low-grade hamstring strain, but selectors are taking no chances © Getty Images

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Matches: Australia 12 - 38 South Africa

James Horwill will miss the Wallabies' coming Rugby Championship Tests against South Africa in Brisbane and Argentina in Perth after precautionary scans on Wednesday revealed that he had strained his left hamstring during training at Ballymore on Tuesday afternoon.

Wallabies selectors have promoted Kane Douglas into the starting line-up alongside Rob Simmons, while Western Force back-rower Ben McCalman replaces the New South Wales Waratahs lock on the bench. Will Genia captains Australia in Horwill's absence from the Nelson Mandela Challenge Plate Test against South Africa at Suncorp Stadium on Saturday, with Ben Mowen appointed vice-captain.

Genia admitted he'd been below his recently best but denied he was struggling with a knee injury. The 46-Test star blamed poor decision-making, opting against testing the defence, rather than a lack of confidence in his reconstructed knee.

"I've spoken to the coaches about that - I probably need to run a little bit more and have a go and provide a bit more of a threat and a spark in that ruck area," he said. "The good thing for me is it's something I do well and naturally anyway. In terms of the whole team attack, it then holds the defenders in tight and creates space outside for guys like Quade (Cooper) and Izzy (Folau). I feel like I'm in good shape and good condition and hopefully full of running come Saturday night."

Wallabies coach Ewen McKenzie backed Genia to step up in Horwill's absence, noting the scrum-half had led the Wallabies on three previous occasions - most recently skippering the side to their 26-19 victory over the Springboks in Perth 12 months ago.

"Will is a proven leader and has had plenty of success as captain for the Queensland Reds and also for Australia," McKenzie said. "He is a player that thrives with the added responsibility and he has always stepped up when given the opportunity to lead. I'm sure this will bring the best out of him.

"It's extremely disappointing to lose James but after discussions with medical staff we felt it was important not to risk further injury when he isn't ready. Scans revealed his injury was on the lower-end in terms of severity and we expect him to be back to full fitness when we embark on our two-game tour of South Africa and Argentina in a few weeks."

Wallabies: Israel Folau; Nick Cummins, Adam Ashley-Cooper, Christian Leali'ifano, James O'Connor; Quade Cooper, Will Genia (captain); Ben Mowen, Michael Hooper, Scott Fardy; Kane Douglas, Rob Simmons; Sekope Kepu, Stephen Moore, James Slipper. Replacements: Saia Fainga'a, Scott Sio, Ben Alexander, Ben McCalman, Jake Schatz, Nic White, Matt Toomua, Jesse Mogg.

Follow live text commentary of the Mandela Challenge Plate Test between Australia and South Africa on Saturday, September 7, from 730pm (AEST), 1130am (SAST), 930am (GMT)