The embattled Brumbies will gamble on a new centre pairing for their do-or-die Super Rugby derby against the Waratahs on Saturday.

Struggling to arrest dwindling home crowds and stay alive in the competition after three straight losses, the Brumbies suffered a setback on Thursday with in-form fullback Pat McCabe ruled out because of a hamstring injury.

Another loss this weekend would make it the Brumbies' worst start - just one win and four losses - since Super Rugby began in 1996.

Coach Tony Rea has reacted by breaking up his centre combination of the first four games - Wallaby Adam Ashley-Cooper and rookie Robbie Coleman.

Ashley-Cooper will revert to fullback to replace McCabe with classy utility Christian Lealiifano ousting Coleman at inside centre. Andrew Smith will be brought in from the Brumby Runners to play outside Lealiifano.

Brumbies officials will be keeping an anxious eye on the crowd at Canberra Stadium even though coach Tony Rea has dismissed suggestions that some local fans could be disillusioned by such a sluggish start - capped by a 52-10 flogging by the Crusaders - and the sacking of his predecessor Andy Friend after just two rounds.

Home matches against the Waratahs have drawn the best average crowd of around 22,400 for the Brumbies.

But attendances this season have been poor, with the first round match against the Chiefs posting the Brumbies' second lowest attendance in 11 years.

"The local fans have been terrific generally speaking," Rea said. "People are right behind the team."

The selection of Lealiifano gives the Brumbies a strong left-and-right kicking option with skipper Matt Giteau from phase ball to help pin the Waratahs back three near their opponents tryline.

"He's (Lealiifano) got a really good football brain and the way he approaches his footy will help this group," Rea said.

Lealiifano was excited about his first start in a year after a serious knee injury early in 2010.

"I'd like to mix up my game a little bit and keep the Waratahs on their toes," he said, expressing a preference for inside centre despite his versatility to play fullback and five-eighth.

"I like more time and a bit of space and definitely hope to take the load off Matt Giteau."

Having swapped positions, Ashley-Cooper has another move on his mind with the Wallabies star to decide by March 31 whether to stay in Canberra or shift to Sydney with the Waratahs from 2012.

Huia Edmonds replaces injured Wallaby hooker Stephen Moore while Julian Salvi moves from the bench to start at openside breakaway with Colby Faingaa switching to blindside in place of the injured Mitchell Chapman.

Brumbies: Adam Ashley-Cooper, Henry Speight, Andrew Smith, Christian Lealiifano, Francis Fainifo, Matt Giteau (capt), Josh Valentine, Ita Vaea, Julian Salvi, Colby Faingaa, Mark Chisholm, Ben Hand, Dan Palmer, Huia Edmonds, Ben Alexander. Res: Anthony Hegarty, Salesi Ma'afu, Peter Kimlin, Michael Hooper, Pat Phibbs, Matt Toomua, Robbie Coleman.

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