Sydney Swans midfielder Callum Mills has undergone shoulder surgery after a Mad Monday celebration gone wrong.

Channel 7’s Mitch Cleary reported on Tuesday night Mills suffered a shoulder injury while wrestling with a teammate during the side’s post-season drinks.

Cleary added Mills was in “severe doubt” to play Round 1 next year. The Daily Telegraph reported later Mills had undergone surgery and was certain to miss the start of the 2024 season.

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Sydney confirmed Mills’ injury on Tuesday night.

“Losing players to injury in the off-season is always disappointing,” Sydney football boss Charlie Gardiner said.

“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.”

SEN journalist Tom Morris on Wednesday reported Mills had “ruptured his rotator cuff – torn it from the bone”, which can be a nine to 12-month injury.

“He was mucking around with Jacob Konstanty when the injury occurred. It was innocuous at the time. Teammates didn’t even notice,” Morris wrote on X.

Fellow SEN reporter Sam Edmund shed more light on how the injury unfolded.

“This is going to be a big story as it unfolds. It’s a disaster. This is a serious shoulder injury for Callum Mills,” Edmund told SEN Breakfast on Wednsday.

“Definitely going to miss Round 1 and I’m told if this (had happened in) March, he most likely wouldn’t play for the year.

“They’re in an establishment in the city of Sydney – a friend of the club, so it’s a private function, no one knows they’re there – and things sort of escalated as they do. There’s a wrestle with a teammate, a few wrestles, the co-captain has got himself into a situation where he’s seriously hurt his shoulder but at the time, I’m told, he’s unaware of the gravity (of his injury).

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“The club didn’t know, he didn’t know for several more days after that the seriousness of it. They’ve gone in, got it checked out … and it’s a serious shoulder injury for the co-captain of the club in a setting that you certainly wouldn’t expect something like that to happen of this severity.

“This is why Charlie Gardner obviously has used words like ‘extremely disappointed’, because it’s rocked the club and it seriously upset the club too.

“He’s had that operation last week, he’s now in rehab and it’s going to be a long rehab.”

Edmund added via X it was a “Mad Monday wrestle gone spectacularly wrong”.

Mills, 26, has played 155 senior games since being drafted as an Academy prospect with pick No.3 in the 2015 draft.

He won the club’s best and fairest award in 2022.

Mills injured himself in 2018 following a training session while throwing an American football with friends, when he landed on uneven ground and fell.