A devastated Mann-Rea's plan to lead the Brumbies into the play-offs now hinges on scan results after he broke down 22 minutes into his appearance for the Gungahlin Eagles on Saturday.

Wallabies captain Stephen Moore (knee) and Siliva Siliva (knee and ankle) are already out for the rest of the season, leaving ex-coal miner Mann-Rea as the man to shoulder the hooking duties in the make-or-break last two games of the Super Rugby campaign.

But in a cruel twist, Mann-Rea will have scans on Monday to determine if he will be fit to play against the NSW Waratahs on the weekend as the Brumbies attempt to keep their season alive.

If Mann-Rea is ruled out, it means Super Rugby journeyman and qualified plumber Luke Holmes will be thrust into a finals-like cauldron at ANZ Stadium in six days, less than a year after retiring from professional rugby.

Regular prop Ruaidhri Murphy looms as the most likely man to fill a bench spot as Holmes' back-up, with director of rugby Laurie Fisher likely to opt for a player who already knows the Brumbies' systems.

But Christian Lealiifano, who played for Tuggeranong against Mann-Rea, warned Super Rugby rivals to write the Brumbies off at their peril.

"It's massive if [Mann-Rea's] out, he's pretty devastated but hopefully it's not as bad as we all think," Lealiifano said.

"We just hope for best for him. Losing three hookers all at the same time is unheard of, but rugby is crazy like that.

"You always back your set up ... it will be no different whoever is there. Everyone will write us off, but that's the best way to play footy - when you're up against it. We enjoy that challenge."

Mann-Rea spent Saturday night icing his hamstring and is clinging to the hope he will be able to take his place in the team, but the Brumbies are in an unheard of scenario of having all three contracted hookers battling injuries at the same time.

Mann-Rea, 33, was the last fit hooker on the Brumbies' 35-man roster after Moore and Siliva both had season-ending operations.

The three-week Super Rugby bye for international rugby has been a nightmare for the Brumbies, with lock Sam Carter also out of action and racing to be fit for the last game of the regular season on July 11.

It could have been worse for the Brumbies if Lealiifano had suffered an injury in the same game as Mann-Rea, but despite needing attention from trainers in the first half the Wallabies centre got through 40 minutes.

The Brumbies need to win both of their games to guarantee a finals berth, but could secure a top-six spot if they can win one of their last two games.

The Waratahs will be riding high on confidence after the bulk of their squad played a  pivotal role in the Wallabies getting a 3-0 series clean sweep in their series against France.

Monster 140-kilogram lock Will Skelton looms as a massive threat to the Brumbies, with the Waratahs' forward pack boasting several Wallabies and a South African Test representative.

Holmes will arrive in Canberra on Monday to start training and, if Mann-Rea is out, will step up having already played at the Waratahs, Western Force and Melbourne Rebels.

Brumbies winger Henry Speight got through his Gungahlin debut unscathed.

"We do need Bongo [Mann-Rea]. It's crazy that all three hookers have gone down recently and Bongo and [Moore] within two weeks of each other," Speight said.

"We're a team that enjoys playing with our backs against the wall. We will rally around each other, it's a tight group and in the past we've been able to step up.

"It's going to be a massive test of our character and unity as a team ... we'll just get behind whoever is in there and fingers crossed for Bongo."