The 24-year-old played in just over half the Scarlets' Magners League fixtures and is now in line to make his senior international debut 12 months after missing out on a place in the squad to tour North America.
"Rob has been around the squad for some time and was unlucky to miss out on the tour to America and Canada with us last summer after being named in the larger initial squad," said Wales assistant coach Robin McBryde.
"He is a tough young prospect and it is only a matter of time before he gets his chance and wins that first cap and if we need him on Saturday he won't let us down."
Powell, who hasn't played for his country since being dismissed from the Wales camp following a disciplinary issue in February, pulled up in training on Tuesday and a scan later revealed a tear to his left thigh muscle.
The British & Irish Lions squad member faces at least four weeks on the sidelines and has therefore been ruled him out of the meeting with the Springboks and the two-Test tour of New Zealand which follows later this month.
Thomas, who featured 15 times for the Dragons in the Magners League this season, playing the full 80 minutes on 13 occasions, will take Powell's place on the plane next week.
Last capped by Wales on the summer tour of Australia in 2007, Thomas has represented three of the four Welsh regions in the Magners League and was in impressive form for the Dragons as they came close to a shock Play-Off spot this season.
"Gavin is an experienced player with over 20 caps who has had some injury problems over the last few years, but he is fighting fit now and has impressed everyone in training," added McBryde.
"It's obviously disappointing to lose someone of Andy Powell's quality, but Gavin now has a chance to make and impact and he, like Rob, won't be taking that chance lightly."
Revised Wales squad vs South Africa, Millennium Stadium, Saturday, 2.30pm
Lee Byrne; Leigh Halfpenny, James Hook, Jamie Roberts, Tom Prydie, Stephen Jones, Mike Phillips; Paul James, Matthew Rees, Adam Jones, Bradley Davies, Deiniol Jones, Jonathan Thomas, Sam Warburton, Ryan Jones (captain)
Replacements: Huw Bennett, John Yapp, Alun Wyn Jones, Rob McCusker, Richie Rees, Dan Biggar, Andrew Bishop
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