Sydney will be without captain Callum Mills until the start of next AFL season, at best, after the Swans' Mad Monday went very bad.
Mills damaged the rotator cuff in his shoulder when he wrestled with a teammate.
According to media reports that player is Jacob Konstanty, drafted at pick 20 last year.
Konstanty is yet to make his AFL debut and is a small forward who prides himself on defensive pressure and tackling.
While the Swans are hopeful the damage is not severe, Mills has undergone surgery and will be out of action for several months.
At worst, he might not play again until midway through next season.
The Swans are fuming about the injury, which only came to light through the media late on Tuesday.
"Losing players to injury in the off-season is always disappointing," Swans football boss Charlie Gardiner said in a statement.
"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."
Mills missed most of the 2018 season after breaking his foot in a bizarre accident.
In May of that year, the Swans announced Mills was throwing an American football with teammates on the way back from a cafe when he tripped and fell.
The 26-year-old, one of Sydney's most important players, has featured in 155 games since his 2016 debut, originally at half-back before moving into the midfield.
Last year's grand finalists, Sydney finished eighth this season and were knocked out by Carlton in the elimination final.
