Triple-premiership forward Breust's right knee twisted awkwardly when he was attempting to lay a tackle in Box Hill's VFL preliminary final loss to Footscray.
Scans confirmed both an ACL and MCL tear, bringing Breust's year to a close and officially ending any hopes of the veteran forward featuring late in the Hawks' bid for this year's AFL premiership.
Sam Mitchell's side will take on arch-rivals Geelong at the MCG on Friday night for a spot in the grand final.
Breust played just eight AFL games this year, kicking four goals, and was used as the substitute five times and subbed off another.
He was Hawthorn's substitute for the final three regular-season games, but was dropped for finals.
Breust, 34, announced in August that he would retire at season's end, and he will finish with 553 goals from 308 games.
Former rookie Breust is a five-time leading goalkicker at Hawthorn and was an All-Australian in 2014 and 2018.
"Despite the abrupt and unfortunate ending, Luke should be enormously proud of committing himself completely across his incredible career, on and off the field, right up until his final moment," Hawks football boss Rob McCartney said in a statement.
"Having played his 308th game in the brown and gold just a few weeks ago, Luke's legacy as one of our club's true greats has been etched into the history books, sitting behind only Michael Tuck and Leigh Matthews for most games played at Hawthorn.
"The club will be there to support Luke through every stage of his rehabilitation, just as Luke has been there to support so many others during his time in the brown and gold.
"Importantly, today we wrap our arms around our much loved 'Punky'."
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