Sydney's 2023 campaign was culled at the hands of the Blues in an elimination final two weekends ago, having qualified for this year's finals series after finishing in eighth position with 12 wins.
According to the Swans, Mills underwent successful surgery last week and is currently in the early stages of his recovery, with a timeline for his return yet to be determined.
The club did not specify the exact nature of their co-captain's ailment.
However, according to Seven reporter Mitch Cleary on Tuesday night, the 26-year-old sustained the worrisome injury after a 'Mad Monday' arm wrestle with a teammate went wrong.
#EXCLUSIVE: Sydney co-captain Callum Mills has been sent for surgery after Mad Monday celebrations went wrong. @cleary_mitch is live with the exclusive details. https://t.co/5zYfOfGqUb #7AFL #7NEWS pic.twitter.com/GYcdExs7l4
— 7NEWS Melbourne (@7NewsMelbourne) September 26, 2023
Sydney Swans EGM of Football Charlie Gardiner lamented the nature of Mills' injury, noting the club's disappointment.
"Losing players to injury in the off-season is always disappointing," Gardiner said in a Swans club statement on Tuesday evening.
"Callum is our captain and an important member of the squad. We are extremely disappointed this injury has occurred.
"Callum is equally disappointed, as you can imagine, but he is recovering well and will get to work on his rehabilitation immediately."
It's not the first time Mills, a 2022 All-Australian and Swans best and fairest, has had a non-footballing mishap during his AFL career.
In 2018, the versatile ball-winner tripped over "uneven land" while attempting to catch a gridiron ball after training, subsequently falling and breaking a bone in his foot. He missed 13 games as a result of the accident.
As no return-to-play timeline has yet been announced by Sydney, Mills' 2024 Round 1 playing status appears in some doubt.