NSW Waratahs have lost their scrum linchpin Benn Robinson and were left to sweat over the fitness of fellow front runner Tatafu Polota-Nau and centre Rob Horne heading into Saturday's do or die Australian Super rugby derby with the Brumbies at ANZ Stadium.

Robinson was ruled out on Friday after a new scan revealed small fractures in his forearm, which was damaged in the Waratahs most recent game against the Crusaders.

"The fractures just haven't healed sufficiently and if he did play, he would have a risk of opening them up again," Waratahs' coach, Chris Hickey told AAP.

Another Test prop, Sekope Kepu will take Robinson's place, with Brumbies' bound Dan Palmer moving onto the bench.

Hickey will wait until Saturday before making a final call on hooker Polota-Nau (shoulder) and Horne (buttock).

He said Polota-Nau, who would be replaced by Adam Freier if scratched, had told him he felt really good following Friday's captain's run, while Hickey wasn't taking any chances with Horne.

"They (the medical staff) said if he ran today, that you just risk flaring it up again, so we are prepared to wait and have a look tomorrow," Hickey said of Horne who would be replaced by Tom Carter.

With between three and five matches remaining in the tournament, Hickey didn't want NSW to rush players back from injury.

Both the Brumbies and Waratahs said they had spent the week focusing more on themselves than each other.

"Ideally you've got to fix your own backyard first and that's probably what we've spent 70 to 80 per cent of the week on," Brumbies captain Stephen Hoiles said.

The Waratahs head into round 11 in fifth spot, three points above the Brumbies.

With a total of around 42 points normally needed to make the finals, the Brumbies (26) cannot afford another loss, while defeat for the Waratahs (29) could do severe damage to their finals aspirations.

"We know now that another loss is pretty much going to get us out of finals contention," Brumbies coach Andy Friend admitted.

The frequent failure of forwards and backs to successfully combine for tries in the opposition 22 has remained a major focus for the Canberra-based franchise.

"Pretty much everything we do up until we get to that last 20 metres has been going pretty well, it's just basically saying to the fellows that when we get into that zone, there's no need to tighten up and adjust," Friend said.

He was wary of the set-piece prowess of the Waratahs.

"They are tough in their set-piece, they like structure in their game, they are very good in their first and second phases, so our real challenge is to make sure we shut them down there," Friend added.

He was confident rookies, centre Andrew Smith and flanker Colby Faingaa would not be overawed each having their first Super start on Saturday.

Test veteran Waugh, who Friend described as a "warrior-like" leader, will enjoy a significant advantage in experience over Faingaa and fellow teenager and Brumbies replacement openside flanker Michael Hooper.

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