McMahon's injury could open doors for the Brumbies in more ways than one, with the hard-hitting back-rower to miss Melbourne's clash against the ACT side in Canberra on Saturday night.

The Brumbies have all but secured Australia's guaranteed finals berth after winning both games on a South Africa and Argentina road trip and the team is oozing confidence after a 39-15 shellacking against the Jaguares.

The Brumbies' return to form could catapult Alcock into Test calculations as Wallabies mentor Michael Cheika weighs up his back-row options.

Cheika will name his mid-year Test squad on Tuesday night, with NSW Waratahs skipper Michael Hooper a guaranteed selection at openside flanker.

But that leaves Alcock, Queanbeyan junior Colby Faingaa and veteran George Smith to fight for a back-up role. If Alcock is picked, it would cap a brilliant year for the journeyman, who is capably filling the hole left by David Pocock.

Cheika is expected to pick several Brumbies in his squad given they are the form Australian team, with Sam Carter, Scott Fardy, Joe Powell, Tevita Kuridrani, Rory Arnold, Allan Alaalatoa and Henry Speight in contention.

Injured prop Scott Sio could also be included, but he is unlikely to be available for the first Test against Fiji on June 10.

Alcock has been in superb form and could be the bolter of the group if Cheika needs another ball fetcher, but Rebels star Faingaa is enjoying his best season in Super Rugby while Smith is a proven international star.

Faingaa was part of the Australian under-20s side with Hooper but has patiently waited for a chance while Hooper's career has boomed.

Carter has been on the Wallabies' outer since Cheika has taken charge, but the Brumbies' skipper has forced his way back into contention.

"Sam has worked on the things we asked him to at the end of last year and I think we've seen some real improvement there," Cheika said.

"It's not necessarily about getting angry, just getting the shoulders down and bringing that physicality. He's added leadership as well, he's running [the Brumbies'] lineout and his work rate for locks in Australia is probably the best."

The Brumbies arrived in Canberra on Monday night after a 34,000 kilometre journey around the world.

They will be on weary legs when they play the Rebels at Canberra Stadium, but they are riding high on confidence and determined to guarantee they will play finals for a fifth consecutive season.

The Brumbies are nine points ahead of the NSW Waratahs and another win would all but snuff out any challenges for their top ranking in Australia.

"We're finding balance in our game now," said Brumbies coach Stephen Larkham.

"We've modified some things in our attack, but we won't get carried away with that. It's about getting the balance right.

"We've tidied up a few things but we'll certainly take a lot of confidence out of the fact our attack worked well on this tour and that's good going forward.

"We made a big number of linebreaks in the last two games and we didn't capitalise on some of those. That's the next step ... but they are good signs as well."

The Rebels beat the Brumbies in a shock result six weeks ago. It was the Rebels' only win of the season and most expected the Brumbies to win given they thumped the Queensland Reds in the previous week.

"We knew this was an important period of the season for us. But the Rebels are our No. 1 priority on the back of what happened last time we played them," Larkham said.

SUPER RUGBY ROUND 15

Saturday: ACT Brumbies v Melbourne Rebels at Canberra Stadium, 7.45pm. Tickets available from Ticketek.