THE SYDNEY Swans' ruck stocks have copped another blow with the news that Callum Sinclair could be sidelined for the next month with a knee injury.

Sinclair hurt his right knee against Carlton in the Swans' six-point win last Saturday, and although he played out the match at the SCG, subsequent scans revealed enough damage to require an operation.

“Callum suffered a little discomfort during and after the game,” Swans football manager Tom Harley said.

“The scan revealed some cartilage had broken off in his right knee, so he underwent arthroscopic surgery on Tuesday morning to remove some of that floating cartilage.

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“We won’t know the full prognosis until he recovers from the surgery, but we are hopeful he will be able to return within a month.”

The loss of Sinclair means the Swans will travel to Perth to take on Fremantle this weekend without their two best ruckmen, with Kurt Tippett already sidelined.

Tippett, who has been placed on the long term injury list, had surgery on his hamstring tendon after the Swans' round 12 loss to Greater Western Sydney, and isn't expected to return until at least round 21.

Toby Nankervis is certain to replace Sinclair and partner fellow youngster Sam Naismith against the Dockers.

The pair has played just 12 games between them, and three each this season.

Nankervis replaced Tippett in round 13 and played two straight games before being dropped for Naismith, who has taken over as the Swans' No.1 ruckman in the past three matches.