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."
Vermeulen missed the playoff games in 2011 and 2012 because of injury and the Stormers struggled without his bullocking runs, strong defence and leadership.
This season Coetzee said they had useful replacements. Nizaam Carr was a No8 and Springbok Schalk Burger, Siya Kolisi and Michael Rhodes were other top class loose forwards.
Goal kicking five eighths Dimitri Catrakilis has been cleared of concussion after being heavily dazed in a collision and missing last week's final pool game. He has been cleared to play Sunday's knockout match and will compete with Kurt Coleman for the starting role.
Hooker Scarra Ntubeni is also recovering strongly to be in selection contention while stormy weather pushed the Stormers' training sessions indoors in the early part of the week.
As the winner of the South African conference, the Stormers qualify automatically for the finals and host one match while the Highlanders host the other match against the Chiefs in Dunedin.
On an accumulated points table, the Stormers would have finished seventh, behind the Crusaders who missed the finals by a point.