The injury, sustained the injury in the weekend match against New South Wales in Sydney, has ruled the Brumbies five-eighth out of rugby for between four and six weeks.
Larkham said today that he would target a return for the final Super 14 round match against the Crusaders, in Christchurch on May 12, but the worse-case scenario of six weeks would see him miss the finals, which begin the following week, should the Brumbies make the last four.
Should Larkham miss the finals, he will be battling to be eligible for Australia's season-opening Test against England at Telstra Stadium in Sydney on June 11.
"There is some hope that I'd be available to play in the final game of the regular (Super 14) season, but conservatively speaking it's more likely that I'd be fit to return for the semi-final if we happen to qualify," Larkham said today.
Larkham limped from the field in the 12th minute of the Brumbies' 37-14 loss to the Waratahs at Aussie Stadium on Sunday night.
The Brumbies, in fourth spot on the Super 14 ladder and facing games against the table-topping Crusaders and 2005 semi-finalist the Hurricanes in the four-match run home, will sorely miss Larkham.
"Losing Stephen for what could possibly be the remainder of the regular season is a big blow to us," Brumbies coach Laurie Fisher said today.
"He's our primary play-caller, and one of our most experienced players, so obviously we can't replace that."
Fisher meanwhile said Larkham's absence would give Gene Fairbanks a chance to settle into the No.10 jersey.
"It's certainly a blow not having Bernie (Larkham) there and it's certainly going to change the way we'd like to play," he said.
"(But) it will certainly be interesting to see how (Gene) goes over the next few weeks, to see if he does have a future at number ten (and) it will certainly give us an indicator for the years to come."
Fisher admitted the Brumbies would play a less expansive game without Larkham directing the attack.
"We certainly won't be able to play with as much of the width as we can get out of Bernie," he said.
"Gene's go to is certainly much more (about) his footwork, his capacity is to be more direct to the line and to get runners a little closer off him."
Adding to the Brumbies woes ahead of the match against the third-placed Hurricanes this weekend are doubts over the fitness of captain Stirling Mortlock and centre partner Matt Giteau.
Mortlock strained his left shoulder during a tackle on Waratahs wing Wendell Sailor last weekend, while Giteau is still troubled with the knee injury he sustained during the Commonwealth Games sevens competition in Melbourne last month.
Mortlock said he was immediately aware he had done something to his shoulder when he clashed with Sailor.
"I actually told the medical staff I needed to get off the paddock, and they were desperately trying to get me off the paddock before I actually tore it," he said.
Fisher said he had to wait for x-ray results on Mortlock.
"Stirling and Giteau are probably 50-50 to play at this stage, so were giving them until Thursday to prove their fitness," he said.
Fairbanks at number 10, oh no Surely the Brumbies have some up and coming 5/8 (leaving Giteau at 12)