Day limped from the field late in Hawthorn's loss to Adelaide on Friday night and returned to Melbourne with his right foot in a moonboot.
Scans confirmed bone stress in Day's navicular, the same injury he had earlier this season, in the same foot.
The gun midfielder required surgery in April and missed 14 games before returning in late July. He had previously had a navicular injury in early 2024.
Day will miss a second consecutive finals series - if the Hawks make it without him - after a complex collarbone injury prematurely ended his 2024 season.
"We know Will has put in a mountain of work to get back to playing, and indeed performing, at the elite level, and we are really disappointed for him," Hawks football boss Rob McCartney said."The club's medical and high performance team have taken extra precautions with Will throughout his rehabilitation this year.Â
"This included very closely managing his workload and consulting external specialists both in Victoria and interstate to help guide his return to play timeline.""Will is an outstanding person who has showcased an incredible amount of resilience, and we know he will engage his rehab with the same attitude."As a club, we will obviously support Will throughout every stage of his rehabilitation and wrap our arms around him as he navigates this tough period."
The Hawks (fifth, 52 points) play Collingwood, Melbourne and Brisbane to finish the home and away season.