The Brumbies flyhalf is set to be sidelined for up to eight weeks with a hamstring injury that leaves him racing the clock to be fit for the Super Rugby AU finals.

The 20-year-old is hopeful of a return after the severe hamstring strain brought his night to a premature end against the NSW Waratahs last week.

There are seven rounds left in the regular season with the undefeated Brumbies tipped to feature in the finals which begin on September 12, with the final taking place a week later.

Lolesio has enjoyed a breakout year to date after winning the race to the Brumbies' No. 10 jersey, emerging as a potential playmaking option for the Wallabies at season's end.

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Brumbies mentor McKellar is set to turn to Bayley Kuenzle and Reesjan Pasitoa to fill the void, with Kuenzle set to start against the Western Force at Leichhardt Oval on Saturday.

"The sun comes up tomorrow and we've just got to get on with it now. [Lolesio] will come back a better footballer if he approaches it with the right mindset, which I've got no doubt he will," McKellar said.

"[A finals return] would be the hope. It's disappointing news, he has had a good introduction into Super Rugby and played well against the [Melbourne] Rebels in the first round of the Super Rugby Australia comp.

"As I said to Noah, injuries are part of the game. It's not a long-term one, so he's got to look at the positives and what he can do to better himself over the next eight weeks.

"At the same time, as injuries always do, it provides opportunity for others. It's up to Kuenzle and or Pasitoa to step up now and take their opportunity, they're two very good young footballers as well."

Kuenzle was the man to take the reins during the Brumbies' last-gasp win over the Waratahs which saw them secure back-to-back wins.

"His point of difference is he is a big body, has a good running game, defensive player, kicks well. He'll take this opportunity. He would have learnt plenty out of Saturday night [against NSW]," McKellar said.

"He is no different to Noah really, Noah has played a few more minutes than him this year but they were both challenging each other along with Reesjan over that pre-season period. Noah got the first opportunity and took it, it's now up to these two to do the same.

"[Kuenzle] will get the first opportunity this week. We've got full confidence in him and Reesjan as well. They're both good young footballers who we will back."

The club was made to wait until Tuesday afternoon for a verdict on Lolesio's injury following scans which revealed the damage was more severe than first thought.

McKellar has another selection headache to contend with as the club monitors James Slipper after he was forced off the field with a concussion against the Waratahs.

It could open the door for Harry Lloyd to make his first appearance in the rebooted domestic competition against his former club.

He may force his way back onto the bench if Slipper is ruled out, which would see Scott Sio return to the starting XV and allow the Brumbies to retain an all-Wallaby front-row.

"If the opportunity comes, I'll be ready. If I get the nod, I'll be ready to go come Saturday," Lloyd said.

"We train at near enough game intensity here. The physicality side of it, I'm ready to go. It's just the knowledge and a bit of the game speed stuff [that I need to get used to].

"Scotty, Slips and Al [Alaalatoa] are the big dogs in Australia at the moment. They're the best of the best, so I think I have definitely had the opportunity to test myself against them.

"After the lockdown period and scrumming against these guys, I'll be ready to go."

SUPER RUGBY AU ROUND FOUR

Saturday: Western Force v ACT Brumbies at Leichhardt Oval, 7.15pm.