Zorko has missed the past fortnight with a quad niggle, and fresh scans have revealed more damage than the initial injury.
"It's a concern, because he's having trouble with a quad injury," Brisbane coach Chris Fagan said on Thursday.
"But he is 37 years of age, so maybe that's got something to do with it.
"He felt sore again at training last week, and scans have revealed he's got more damage there than he's got previously.
"It's just one of those things in the rehab process. All rehab processes don't go perfectly unfortunately.
"He was looking good, and then late in the last training session last week he felt it again, and we re-scanned it and it wasn't great news."
Zorko's absence has coincided with a big drop in form by Brisbane, who have lost to Geelong (41 points) and GWS (78 points) over the past fortnight.
The Lions will be aiming to bounce back in Saturday's night's clash with Fremantle, who are set to regain skipper Alex Pearce (knee) but may be without Hayden Young again due to concussion.
Sydney are set to make at least three changes from last week's 27-point loss to Geelong after Mills joined Tom Papley (calf, six weeks) and Malcolm Rosas Jr (ankle, 1-2 weeks) on the injury list.
Mills barely trained on Thursday, with the 29-year-old set to be put in cotton wool in the hope he will return for next week's clash against St Kilda.
Swans coach Dean Cox will resist the urge to rest ruckman Brodie Grundy this week despite his heavy workload, but he was feeling for Papley.
"To hear that you're out for a little bit of footy isn't the news you want to receive as a player, as a coach and as a football club," Cox said.
"Malcolm's was better news than probably what we expected straight after the game."
Richmond's injury list featured 19 players when it was released this week.
Spearhead Tom Lynch was the latest player to be added to it after he copped a hit to the throat during last week's 18-point win over Essendon.
