Powell suffered a torn thigh muscle in training on Tuesday, ruling him out of the tour, with McCusker stepping up to the bench and Newport Gwent Dragons flanker Gavin Thomas joining up with the squad.
Wales forwards coach Robin McBryde said: "It's bad news for Andy, but from our side of things, it gives another couple of individuals an opportunity, namely Rob McCusker who will step up to the bench on Saturday and Gavin Thomas from the Dragons who will join us for the tour."
"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 McCusker, won't be taking that chance lightly.
"Gavin is an experienced player with more than 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.
"Rob has been around the squad for some time, and he was unlucky to miss out on the tour to America and Canada with us last summer after being named in the larger initial squad.
"He is a tough young prospect, and it is only a matter of time before he gets his chance and wins that first cap. If we need him on Saturday he won't let us down."