Jarrad McVeigh will miss at least the next month of football after straining his calf in Saturday's NEAFL win over Southport.
Meanwhile, superstar forward Lance Franklin's 300th game is still a few weeks away after his hamstring strain.
McVeigh made his return to the field in Saturday's NEAFL win over Southport and was one of his side's best with 21 possessions, eight tackles and a goal, but strained his calf in the final term.
Scans confirmed the 34-year-old will miss at least four weeks with the latest setback.
It was the former co-captain's first match since Sydney's round 12 win over West Coast at the SCG, when he suffered a hamstring injury.
That match was just his second game back after missing seven weeks with quad strain.
McVeigh has played just five games this year and has battled with soft tissue injuries frequently over the past three seasons.
Swans coach John Longmire revealed the bad news on Monday but said it's too early to speculate on what it means for McVeigh's career.
"We haven’t had a chance to talk about it," he said.
"He knows football really well 'Macca', so we'll sit down over the next week or so and have a chat about it and see where he goes from here.
"He's clearly disappointed that it's another soft tissue injury.
"He's a pretty resilient sort of bloke, I spoke to him Sunday night, and he's already started his rehab program.
"But I think this is even testing him a bit."
McVeigh is one of the game's most professional players in terms of his preparation and rehabilitation from injuries, and Longmire said he has no doubt that the dual club champion and All Australian wants to play senior footy again this season.
"That would be his first thought," he said.
"With these players, particularly those who get (to play) over 300 games, they're not only talented players but they're absolute competitors, it's very hard to turn to competitive tap off.
"His first instinct is always, 'righto what do I have to do to get back', because that’s what he's always done."
Superstar forward Lance Franklin is up and running as part of his own rehabilitation from his latest hamstring injury.
It was initially expected that the four-time Coleman medallist could be fit to play his 300th game against Geelong at the SCG on Sunday week, Longmire virtually ruled out that possibility.
Franklin is more likely to return against Greater Western Sydney at Giants Stadium in round 21, or the following week against Port Adelaide in Adelaide.
"We've just got to make sure we get a bit more work and training into him before we worry about him playing games," he said.
"We'd like to get him into football training by next week, but we're not going to push him to get back at the risk of another re-injury."
Ruckman Callum Sinclair is also running and continuing his strength work in a bid to get back from his serious shoulder injury.
The big man is expected to increase his training over the next fortnight, but if his twice-dislocated shoulder doesn't respond to his rehabilitation, he could be forced to undergo season-ending surgery.