The defender could also potentially miss the entire 2027 season after he went down with a serious knee injury during Sydney's 53-point upset of the Brisbane Lions.
Mills' right knee gave way as he attempted to corral Will Ashcroft near the boundary, late in the first quarter after an understrength Swans outfit burst to a 40-8 lead.
Swans coach Dean Cox was already expecting the worst in the aftermath, with scans on Monday confirming the injury.
Mills will meet with a specialist to determine a date for surgery.
It is the latest setback for the luckless 29-year-old, who missed the Swans' 2024 grand final loss with a hamstring strain he picked up before the preliminary final.
Mills had played just seven games in 2024 after hurting a shoulder in a "Mad Monday" wrestling contest with a teammate.
This year was the first time the defender had played more than 20 games since Sydney's 2022 grand final run.
The upset victory against Brisbane followed a tumultuous few weeks in the fallout from the Pullman Hotel scandal.
Mills has earned significant praise for leading the Swans through the turmoil, which past players have admitted shattered the club's famed "Bloods culture''.
Star players Isaac Heeney, Chad Warner and Nick Blakey - All-Australian contenders before they were banned from being selected - are among the five serving club-imposed bans.
Important tagger James Jordon and defender Riley Bice are the other Swans who won't play again this season.
Cox's selection conundrum is further complicated with key forward Joel Amartey (Achilles) and winger Justin McInerney (hamstring) nearing full fitness.
Hayden McLean - playing as cover for Amartey - slotted three majors against the Lions, with fellow talls Charlie Curnow (five goals) and Logan McDonald (six) also busy.
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