The 20-year-old will likely miss the first seven rounds of the campaign, according to a Roar statement that withheld specific details of the setback.
"Dimitri Valkanis is expected to be sidelined for an extended period following an injury sustained during training last week and is working towards a December return," the club said on Thursday.
"The 20-year-old has been assessed by the club's medical team and has commenced his rehabilitation programme."
Valkanis reportedly picked up a hamstring injury during the Australia Cup clash with Sydney FC in August, before it worsened at a training session later on.
Brisbane's announcement arrived a day after Valkanis' father, head coach Michael, said he didn't want to dwell on last season's injury-riddled list.
"Getting injuries in that period ... we didn't really recover," Valkanis senior told journalists on Wednesday.
"I don't like dwelling on last season ... We want to get back, get the fans excited again".
The Roar finished second bottom last season, winning just six of their 26 games - one more than wooden spooners Western Sydney.
But despite a troubled campaign, defender Valkanis showed promise and inked a new three-year deal.
He played 23 games and was crucial with 22 blocks, 16 intercepts and three goals.
It earned him the best young player award at Brisbane's annual gala in June, a night when the club's sporting director James Gow stated the importance of having Valkanis in the side.
"Dimitri is a key signing and an important piece of the puzzle," Gow said.
Valkanis will miss the Roar's season opener at home to Perth Glory, who finished one spot and five points above them in 2025/26, on October 23.
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