Ashcroft, this season's Rising Star favourite, clutched at his right knee during the final quarter of Saturday's contest at the Gabba and limped off the field.
Scans have since confirmed Ashcroft ruptured his anterior cruciate ligament.
"We are all hurting for Will because you never want to see any player experience a serious injury, especially a player in their first season," Lions football boss Danny Daly said.
"Will is super professional in everything he does, so we know he will do everything he can to recover from this.
"Our main priority is for everyone at the club to wrap their arms around Will and do everything we can to support him and his family."
Ashcroft's injury took the gloss off Brisbane's 9.10 (64) to 7.11 (53) win over Geelong, which kept their hopes of a top-two finish alive.
"It's dampened my mood ... I don't like to see that for any player at any club, anytime," Lions coach Chris Fagan said post-match.
Ashcroft, the son of Brisbane premiership great Marcus Ashcroft, has played all 18 matches so far in his debut season.
The 19-year-old, who averages 22 disposals a game, could still win the Rising Star and is among the contenders for goal-of-the-year after a stunning effort against Fremantle in April.
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