"It's the left shoulder that I've picked up the injury on, not the one that I had surgery on earlier in the season," he said.
"We think it's not too serious, just an impingement in the muscle that gives a sharp pain. I've been advised to rest for this game but hopefully I should be in contention for the Dragons match on Tuesday."
The injury forces Powell out of the Borders game and sees Xavier Rush coming off the bench to play at No.8 with Mark Lewis moving to blindside flanker. Rhys Shellard moves up to the bench.
The Blues now have a team showing eight changes from the team that defeated Ulster last Friday. In the forwards Robin Sowden-Taylor, Deiniol Jones, Scott Morgan and Gary Powell all return to the starting line-up. Gary Powell comes in for Taufa'ao Filise who broke a bone in his hand in the win against Ulster.
Filise will be out for the rest of the season. Scott Morgan will play in second row as he did in the home win against Borders in January.
In the backs Ed Fairhurst, Nicky Robinson, Tom Shanklin and Mosese Luveitasau all start with Nick Macleod moving to fullback. With big games against Welsh rivals the Dragons and the Scarlets next week Ben Blair, Marc Stcherbina, Chris Czekaj and Mike Phillips are all rested.
Coach Dai Young says the Blues have a real title chance, as they trail leaders Leinster by seven points but with a game in hand.
"We've got to win every game, there's a lot of points to play for," Young said. "We've never won up in the Borders, it's not easy up there, so we've got to continue this momentum now."
Blues: Nick Macleod; Rhys Williams, Jamie Robinson, Tom Shanklin, Mosese Luveitasau; Nicky Robinson, Ed Fairhurst; Gethin Jenkins, Rhys Thomas, Gary Powell, Deiniol Jones, Scott Morgan, Mark Lewis, Robin Sowden-Taylor, Xavier Rush (Capt)
Replacements: Scott Roberts, Gareth Williams, Bradley Davies, Rhys Shellard, Mike Phillips, Dafydd Hewitt, Tom Riley