Sydney fear Alex Johnson has torn the ACL in his "good" right knee and will require a sixth reconstruction in a shattering development for the luckless backman.
Johnson limped from the ground in the arms of the trainers during the first quarter of Sunday's win over Melbourne at the MCG.
"It doesn't look great. I'm not too sure," Johnson told the Seven Network.
"It's sort of similar to my first one but we'll see how we go from here."
The 26-year-old's knee buckled underneath him when he changed direction while pursuing Jesse Hogan in the forward pocket.
Johnson was clearly emotional while being treated on the interchange bench before joining his teammates for their quarter-time huddle.
He's undergone five reconstructions among a total of 12 knee operations but last week returned to play his first game since the 2012 grand final - a period of 2136 days.
Johnson's latest injury soured an otherwise triumphant day for Sydney, who also lost Nick Smith to a hamstring issue but showed tremendous grit to post a nine-point win.
"It's a strange feeling for everyone, to be honest," coach John Longmire said.
"You have a great win and you really reach down into the depths of your character for that win, and we were thrilled by it ... but you come off the ground and Alex has got a suspected ACL on his good side, so everyone's digesting that, including Alex."
Johnson is a popular and much-admired player at Sydney and Longmire said it had been tough for both players and coaches to remain focused on the job at hand.
Longmire hoped the fortitude Johnson had shown throughout his career would continue to inspire his teammates.
"He's shown a remarkable amount of resilience, Alex ... the footy world knows that and none more than his teammates," Longmire said.
"We showed a form of that resilience tonight after quarter-time when he went off.
"I guess a bit of that might have rubbed off and it needs to continue to rub off for the rest of the year."
Johnson has managed just 47 AFL games since being picked by the Swans in the 2010 draft.
He was delisted in 2017 but the Swans then used their final selection in the rookie draft to keep him on their books.