The gun midfielder will miss the season start after dislocating his right shoulder during training at the club's new Dingley home on Thursday.
Hawthorn have yet to determine a return timeline for Day, who has at least escaped bone damage.
But the injury is still a sucker punch for the 24-year-old and the Hawks, given Day had been limited to just six games last year through injury.
"This is a disappointing setback for Will, and we really feel for him at this time," Hawthorn football boss Rob McCartney said in a club statement.
"Will is one of the most resilient people I know, and he has done everything possible to return to full fitness already this off-season."
Day was already on a modified pre-season program after suffering a season-ending right foot injury - a bone stress in his navicular - in August.
The gun midfielder had required surgery in April and missed 14 games before returning in late July.
He managed just one more game following his return, having previously had a navicular injury in early 2024.
A complex collarbone injury prematurely ended his 2024 season with Day having now missed Hawthorn's last two finals appearances.
Day isn't the only star under an injury cloud this pre-season, with Melbourne ruck Max Gawn fracturing his finger during training.
The Demons skipper should be fit in time to represent Victoria against WA in the State of Origin clash on February 14.
But fellow Melbourne ruck Tom Campbell will be out of action for a month after injuring his neck in an "innocuous" collision at training.
Campbell will be in a neck brace, with the club to consult a neurosurgeon to determine his rehabilitation program.
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