The Brumbies are bracing for the Pretoria Bulls to unleash their fury at Loftus Versfeld on Saturday night after coach John Mitchell blasted his side's "softness" in Argentina last week.

McKellar says the Brumbies need measured physicality to match the Bulls and wants the forward pack to rise to the challenge without sacrificing discipline.

The Brumbies conceded 20 penalties last weekend, including Arnold being sent to the sin bin and then sent off for a high tackle in the second half.

The towering lock has been in career-best form and has been leading the way for the Brumbies, despite the side slumping to five consecutive defeats.

"As a pack, we've got to bring the physicality. Rory has been leading the way with and without the ball for a number of weeks," McKellar said.

"I think he's been playing as good as he's ever played, so as a group we have to step up and replace that. We've got to make sure that from 1-8 and the bench when they come on, that they bring that energy and raw aggression that Rory has been producing.

"It's in the individual mind set, making sure you bring intent to everything that you do. If you're relying on a speech or conversation to get ready to go, you shouldn't be there in the first place.

"We need to understand that we need to bring a lot of physicality ... the Bulls are going to come out firing, they'll be wanting to respond after what John Mitchell said."

McKellar has called on spring tour Wallaby Blake Enever to replace Arnold in the second row, while Lachlan McCaffrey will start at blindside flanker to replace Rob Valetini.

They're the only two changes to the starting XV, with the Brumbies desperately trying to avoid another loss and equalling the worst losing streak in the club's history.

Inconsistency has been their biggest problem this year. They have shown glimpses of strong attacking rugby this year, but have been plagued by brain snaps and poor decisions.

The mounting penalty count against the Johannesburg Lions last week cost them any chance of victory and McKellar says they must adapt better to referee interpretations.

"But the thing is, the boys have been excellent this week. We've seen progression, without a doubt," McKellar said.

"There was progression from a performance point of view and what we're trying to do on and off the field. We understand this is a results industry, but it's what we from day to day that will get us a result on Saturday.

"The attitude and the vibe in the group is top shelf. It really is, I can't question that at all. It's been like that for much of the year ... touring allows you to bring a group together."

The Brumbies moved to a multi-million dollar former soccer World Cup high-altitude training base in the hope of sparking form and finding confidence.

Christian Lealiifano will continue at flyhalf and the skipper could force his way into the Wallabies squad if he shines against the Bulls.

McKellar described Lealiifano's effort last week as "without any shadow of a doubt" the best game of his comeback season after being diagnosed with leukaemia in 2016.

Wallabies spots will go on the line in the game against the Bulls, with Test coach Michael Cheika to name a squad next week for the three-match series against Ireland.

Lealiifano is being considered as a back-up flyhalf or inside centre to give the Wallabies playmaker depth.

SUPER RUGBY ROUND 15

Saturday: Pretoria Bulls v ACT Brumbies at Loftus Versfeld, 11.05pm. TV time: Live on Fox Sports.

Brumbies: 15. Tom Banks, 14. Henry Speight, 13. Tevita Kuridrani, 12. Kyle Godwin, 11. Andy Muirhead, 10. Christian Lealiifano, 9. Joe Powell, 8. Isi Naisarani, 7. David Pocock, 6. Lachlan McCaffrey, 5. Sam Carter, 4. Blake Enever, 3. Allan Alaalatoa, 2. Folau Faingaa, 1. Scott Sio. Reserves: 16. Connal McInerney, 17. Nick Mayhew, 18. Les Leuluaialii-Makin, 19. Richie Arnold, 20. Tom Cusack, 21. Matt Lucas, 22. Wharenui Hawera, 23. Andrew Smith.

Bulls: 15. Warrick Gelant, 14. Travis Ismaiel, 13. Jesse Kriel, 12. Burger Odendaal, 11. Johnny Kotze, 10. Handre Pollard, 9. Ivan van Zyl, 8. Marco van Staden, 7. Nic de Jager, 6. Roelof Smit, 5. RG Snyman, 4. Jason Jenkins, 3. Conraad van Vuuren, 2. Jaco Visagie, 1. Trevor Nyakane. Reserves: 16. Adriaan Strauss, 17. Mox Mxoli, 18. Mattys Basson, 19. Ruben van Heerden, 20. Thembelani Bholi, 21. Andre Warner, 22. Manie Libbok, 23. Divan Rossouw.