The 34-year-old, who last month announced he will retire at the end of the season, was playing with Box Hill in the VFL on Saturday when his right knee twisted underneath an opponent in a tackling attempt.
Breust lay on the ground for a period before he was carried from the field by two trainers while still in pain and could not put any weight on his right leg.
A three-time premiership star, Breust has kicked 553 goals in 308 games in a brilliant 15-year career.
The two-time All-Australian was the Hawks' substitute in their last three home-and-away games this season but was overlooked for finals wins over GWS and Adelaide.
The 34-point trouncing of the Crows in Friday night's semi-final booked Hawthorn's first preliminary final since 2015.
Sam Mitchell's side will take on arch-rivals Geelong at the MCG next Friday night for a spot in the grand final.
The Hawks are sweating on small forward Jack Ginnivan's availability because of a tight hamstring.
If Ginnivan is unavailable to face the Cats it could have opened the door for Breust to return.
Star forward Jack Gunston is also battling hamstring soreness and rebounding defender Jarman Impey was also hampered against Adelaide.
Key forward Calsher Dear missed the semi-final because of a hamstring strain and the Hawks are yet to put a clear timeline on his return.
Dear's replacement Mitch Lewis had little impact against the Crows and was substituted out of the game.
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