As veteran Dayne Zorko nears full fitness, former West Coast co-captain Allen faces 12 to 14 weeks on the sidelines with a foot fracture.
The key forward injured his right foot late in the last quarter of the reigning premiers' round-seven win over Adelaide and is scheduled to have surgery this week.
Allen's final season at the Eagles was cut short due to knee surgery, with the 27-year-old managing just 12 games.
The key forward had damaged his calf and achilles in West Coast's round-14 loss to Carlton, before opting to have patella tendon surgery to fix an ongoing issue with his left knee.
He had also missed the entire 2022 season with a foot injury.
Fellow forward Eric Hipwood is already sidelined and is not expected back until halfway through the season.
Key defender Darcy Gardiner (shoulder) is at least three weeks away, while midfielder Jarrod Berry (calf) is sidelined for two to three weeks.
Featuring in all seven of Brisbane's games so far, Allen has kicked 12 goals to help the fifth-placed back-to-back premiers to a 4-3 start this season.
Zorko (calf) has been listed as a test to face Essendon at Marvel Stadium on Saturday, while defender Noah Answerth works through concussion protocols.
Isaac Kako also suffered an injury during the Anzac Day clash, and the Bombers young gun will be ruled out in the "short-to-medium term" after suffering a stress issue in his back.
St Kilda will give livewire forward Liam Ryan "every opportunity" to prove his fitness for Saturday's meeting with Carlton after he suffered a corked hip.
Ryan was admitted to hospital late on Sunday night as a precaution after the Saints recruit injured his hip in the second quarter against his former side West Coast.
"Liam was discharged this morning (Tuesday) and has been at home resting with his family," Saints football manager Lenny Hayes said in a club statement.
"He received a pretty significant knock to the hip, but we'll give him every opportunity to play this weekend pending his recovery."
Carlton are hopeful of regaining forward Harry McKay (concussion) and ruck Marc Pittonet (hand).
Fremantle expect injury-prone ruck Sean Darcy to miss three to five weeks after another soft-tissue setback.
Darcy is managing another flare-up of the calf he strained in January.
Ruled out of the round-six derby, the 27-year-old was building his way through concussion protocols when he hurt his calf in a rehab session.
It is just the latest setback among hamstring, calf, knee and ankle issues for Darcy, who had played just 44 games across three seasons before this year.
Fremantle face the Western Bulldogs at Marvel Stadium on Friday night.
Collingwood will welcome back captain and star defender Darcy Moore (knee) for Thursday's match against Hawthorn but lose key forward Tim Membrey for at least a month.
Membrey was able to play out the club's Anzac Day win over Essendon before subsequent scans confirmed a hamstring strain.
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