SYDNEY Swans co-captain Kieren Jack says lingering doubt over his troublesome knee derailed his bid to return for Saturday night's semi-final against North Melbourne.

Jack says fear of re-injuring his knee early in the final was paramount in deciding not to risk himself for the knockout final against the Kangaroos.

"There is still some soreness there. I don't want to come into the team and compromise getting injured again early," he told his club's website on Friday.

"It's a real tricky one. I could potentially play but the risk is too big of re-injurying it. I'd be filthy if I cost the team."

Jack, who missed the Swans' qualifying loss to Fremantle last week after being hurt in round 23, said he was uncertain whether he could have played an entire final.

"There is too much on the line if it went pear-shaped early," he said.

"Unless you are really sure you can come through, it's not worth it.

"It was touch and go. In the end, common sense prevailed.

"It's frustrating because you are so close, you're just that little bit off performing at that level that warrants your place in the team."