Gavin Thomas, the Dragons flanker who was last capped by Wales on the summer tour of Australia in 2007, has been called up from head coach Warren Gatland's reserve list to full squad duty and replaces Powell on the tour to the Southern Hemisphere.
"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.
"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.
"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."
Wales physiotherapist Mark Davies added: "Andy has a grade two injury to his left thigh which effectively sidelines him for a minimum of four weeks and leads to his omission from the tour."
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