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Ioane broke his thumb in the opening match against Italy and had surgery but remained with the team on the chance he would return in time for the knock-out phase.
"It's great for Digby and good for us when you look at the circumstances of Drew (Mitchell) going home," Deans told reporters, referring to winger Mitchell's serious hamstring injury which ruled him out of the tournament.
"(Ioane) makes things happen. He has had a good work rate so it'll be great for him to be out there."
Deans had several other injury concerns that forced him to play number eight Radike Samo on the wing in their final pool game against Russia but had virtually a full squad to choose from for the clash with the Springboks.
"It's good timing," Deans said.
"It's preferable than the other. We have had our challenegs and the boys have come back on stream."
Pat McCabe, who partially dislocated his shoulder in the Wallabies' pool match against the United States, has been named to start at inside centre, while centre Anthony Fainga'a, who was knocked unconscious in the same match, was named on the bench.
Utility back Berrick Barnes has been dropped to the bench despite a lively two-try performance in the Wallabies' 68-22 victory over Russia in their final pool match.
"(Replacement) Berrick (Barnes) has been ill but he'll have a kick tonight and should be right for the weekend," Deans said.
Barnes's illness had not been a factor in his demotion, the New Zealander coach clarified. McCabe was preferred to start as he would provide a direct approach that would challenge the Springboks defensive line early on, while Barnes would offer a more controlled, deft game later, he added.
Lock Dan Vickerman has replaced Nathan Sharpe and will partner captain James Horwill in the second row. Deans said the decision had been tight.
"There is not a lot in it. We are blessed there," he said.
"Sharpey's experience contributed to his involvement (on the bench). Two good men, two blokes who have been through this before and understand (the pressure)."
Team: 15-Kurtley Beale, 14-James O'Connor, 13-Adam Ashley-Cooper, 12-Pat McCabe, 11-Digby Ioane, 10-Quade Cooper, 9-Will Genia, 8-Radike Samo, 7-David Pocock, 6-Rocky Elsom, 5-James Horwill (captain), 4-Dan Vickerman, 3-Ben Alexander, 2-Stephen Moore, 1-Sekope Kepu
Replacements: 16-Tatafu Polota-Nau, 17-James Slipper, 18-Nathan Sharpe, 19-Ben McCalman, 20-Luke Burgess, 21-Berrick Barnes, 22-Anthony Fainga'a