The seventh ranked Stormers must survive their sudden-death playoff without warrior-captain Duane Vermeulen.
The No8 has been troubled by neck problems and, while neuro-surgeons cleared him of structural damage, he has not regained enough mobility to lead his side out against the Brumbies.
"Duane has been improving a hell of a lot, but I don't think it's enough for the weekend," coach Allister Coetzee told the SuperSport website.
"I have ruled him out of this weekend's game but I am hopeful that there is a chance he will be able to play in later games in the knockout phase should we get through the quarter-final."
"We don't want to set him back by playing him when he isn't 100 per cent right."