Sinclair, who has missed only one game this season, suffered a knee injury in a marking contest with Carlton ruckman Matthew Kreuzer last weekend.
He will definitely miss the Swans clash with Fremantle in Perth next Sunday and probably multiple matches after scans revealed a fragment of cartilage had broken.
“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,” Swans’ general manager of football Tom Harley said on the club’s website.
“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.”
Tippett has been sidelined for five matches after having surgery on a hamstring tendon and is on track to return in round 22.
Sinclair played a secondary role in the ruckwork in recent weeks behind 24-year-old Sam Naismith who has played the last three games.
Naismith has played just four career games.
With Sinclair out, Sydney could call up another relatively inexperienced ruckman in 21-year-old Toby Nankervis, who has played just three senior games this season and eight in total.
Nankervis was among the best in his Sydney’s NEAFL side last weekend.
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