In other injury news around the league, star GWS defender Sam Taylor will miss four to six weeks with a fractured toe, while his teammate Brent Daniels has succumbed to an adductor injury.
Carlton's fading finals hopes have taken another hit, with key forward Harry McKay undergoing knee surgery on Tuesday.
Sydney are hopeful of welcoming back Tom Papley and Errol Gulden for Saturday's clash with Port Adelaide, while Collingwood duo Pat Lipinski (concussion) and Brayden Maynard (plantar fascia) could also return this week.
Allen suffered his painful-looking injury while sprinting behind play during Sunday's 34-point loss to Carlton.
The 26-year-old immediately pulled up lame and slumped to the turf before being gingerly helped off the field by two trainers.
West Coast released a statement on Tuesday afternoon revealing Allen had suffered damage to his left calf and minor damage to his Achilles, but there had been no rupture.
They say he will miss for the medium term, with a more precise timeline to be given in the coming weeks.
"We're relieved Oscar has not ruptured his Achilles tendon, which aligns with our medical team's initial assessment, and the good news is that surgery won't be required," West Coast's football manager Gavin Bell said in a statement.
Although Allen's injuries aren't considered serious and won't require surgery, it remains to be seen whether Allen will be able to make it back again this year.
Even if he does return to full fitness in time for the last few rounds of the season, West Coast will be taking a huge risk if they play him.
Allen is a restricted free agent who is being heavily chased by Brisbane and Hawthorn.
The Club has released the following update on the injury sustained by Eagles Co-Captain Oscar Allen.https://t.co/ZmqAnB89xK— West Coast Eagles (@WestCoastEagles) June 17, 2025
The deals currently in front of him reportedly dwarf what Allen is being offered by West Coast.
If Allen decides to leave, West Coast are likely to receive tier-one compensation from the AFL.
With West Coast (1-13) hot favourites to finish last on the table and get the No.1 draft pick that comes with it, their tier-one compensation for losing Allen would be pick No.2.
Even before Allen got injured, West Coast were being urged to wrap their star forward in cotton wool for the rest of the season to avoid him picking up a serious injury that could affect his worth on the open market.
Meanwhile, Giants defender Taylor will wear a moon boot to protect his fractured toe for the next two weeks before beginning to rebuild his training loads.
Taylor suffered the injury during last week's upset win over Brisbane.
Giants duo Toby Greene and Jake Stringer could return for Sunday's clash with Gold Coast if they make it through main training on Friday.
The Blues will be without McKay for four to six weeks, while fellow key forward Charlie Curnow faces a test for Saturday's game against North Melbourne after he had calf tightness last weekend in the win over the Eagles.
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