Dane Haylett-Petty will miss another Rebels match, with a neck concern.
Haylett-Petty suffered a concussion earlier in the season, but more consultation has revealed that he has had a neck injury, coach Dave Wessels said.
"I think at first we thought it was concussion early on but it's actually, through the specialists, they've said it's a neck problem," he said.
"He's probably at about 90 per cent and he's such an important player for us and for the Wallabies, he's one of those guys that we won't risk this weekend unfortunately."
Wessels said he expected Haylett-Petty to return next week, against the Sunwolves, and was still having an influence despite his injury.
"Be nice to have him but a real compliment to him is I think he's been really active this week.
"I think a lot of the leadership group have - he's done a lot of work behind the scenes to prepare the guys for the game, so I'm pleased that despite the fact he's not on the field, he's made a big impact on the environment this week."
Wessels has looked to youngster Semisi Tupou to bring some spark, with the back starting on the wing, pushing Tom English to outside centre, with Reece Hodge at 12, and Billy Meakes on the bench.
The Rebels have lost five matches in a row after a hot start to the season and Wessels said they had to return to the elements that helped them find their early successes.
"I think the Brumbies, they're very good at certain things, the Brumbies," he said.
"I think what we have to recognise is what are they going to bring and how are we going to stop that.
"One of the ways we want to stop that, which we probably moved away from, is doing some of the things that we do well at real intensity and real speed.
"Our focus this week has been, again, on doing a lot of the things that worked for us in the early part of the season, just with intensity.
"That's probably been missing over the last couple of weeks and probably some detail that's been missing in those things over the last couple of weeks."
The Rebels face the Brumbies on Saturday night at GIO Stadium, kicking off at 7:45pm AEST, LIVE on FOX SPORTS and via RUGBY.com.au RADIO.
Rebels team to face Brumbies
IN: Harrison Goddard, Sam Jeffries
OUT: Will Genia, Sefa Naivalu
1. Fereti Sa’aga
2. Anaru Rangi
3. Sam Talakai
4. Geoff Parling
5. Adam Coleman (capt)
6. Angus Cottrell
7. Colby Fainga’a
8. Amanaki Mafi
9. Michael Ruru
10. Jack Debreczeni
11. Marika Koroibete
12. Reece Hodge
13. Tom English
14. Semisi Tupou
15. Jack Maddocks
16. Nathan Charles
17. Tetera Faulkner
18. Jermaine Ainsley
19. Matt Philip
20. Sam Jeffries
21. Lopeti Timani
22. Harrison Goddard
23. Billy Meakes