A "SIGNIFICANT" knee injury to in-form defender Tom Jonas has soured Port Adelaide's 36-point victory over St Kilda at the Adelaide Oval.

Jonas, who would be in the mix for a spot in the Virgin Australia All Australian team, hobbled from the ground after being tackled at the 13-minute mark of the third quarter.

Power coach Ken Hinkley has already forecast he will miss weeks.

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"He didn't finish the game so he's obviously got a significant injury that might put him out for a little bit of time," Hinkley said.

"We'll wait and see. We don't know exactly what that's going to look like."

Port's SANFL side plays on Sunday, giving the club's fringe defenders a chance to press their claims for selection.

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Jack Hombsch has been called upon as a replacement a couple of times this season but forgotten defender Matthew Broadbent, former Sun Trent McKenzie and Jarrod Lienert will be considered as well.

"I reckon they'll be trying to do his scans to make sure there's something wrong," Hinkley joked.

"We've got some players there who are hopefully going to be able to fill the spot for Tommy – let's hope it's not a long absence."

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Jonas has had a frustrating run with injuries, missing round 13 and round 15 with hamstring issues.

But while disappointed to lose his best defender again, Hinkley said the setback was all part of the game.

"We've seen injury after injury after injury at the moment in the competition; all 18 teams are dealing with significant injuries," he said.

"We are going to have to cop a couple along the way like every side does, it's just part of the game - it's a brutal game."