Jordan De Goey has been ruled out of Collingwood's blockbuster showdown with Carlton at the MCG due to groin soreness.
The 28-year-old Magpies star could be sidelined for an extended period, beginning with Friday night's rivalry match in front of an expected crowd of more than 90,000.
Fellow premiership midfielder Tom Mitchell will also miss the clash with a foot issue.
De Goey didn't feature in Collingwood's main training on Wednesday, while Mitchell failed to complete the session.
"He's got tight groins so we're just going to back him off a little bit and hopefully get those things settled down," Collingwood coach Craig McRae said.
"Who knows when he's available? Ideally next week.
"The reality is (the injury) has come reasonably quickly, so my expertise - which is none - suggests that ... if it comes on quickly, it may go away quickly."
Mitchell was subbed out of Collingwood's Anzac Day draw with Essendon, with McRae saying the former Sydney and Hawthorn player has been dealing with a plantar fasciitis problem.
"He's been struggling with it a bit ... we're trying to get him ready to play and he couldn't get through (the session)," McRae said.
The coach threw up names including premiership player Oleg Markov, VFL on-baller Fin Macrae, last week's sub Harvey Harrison and uncapped Lachie Sullivan as potential replacements for De Goey and Mitchell.
The Magpies settled on mature-aged recruit Sullivan - just 10 weeks after he joined the club.
The 26-year-old had been overlooked in drafts since he was first eligible in 2015, but earned a spot at Collingwood following consistent seasons in the VFL with Port Melbourne and Footscray.
"He was VFL best-and-fairest at the Bulldogs last year, a really quality player at that level, and he hasn't missed a beat since he's been here," McRae said before Collingwood confirmed Sullivan's selection.
"We've had him close as an emergency for a few weeks now."
Collingwood will enjoy three consecutive six-day breaks after facing Carlton, bringing more rest for over-worked players.
Dual premiership veteran Steele Sidebottom struggled early against the Bombers on Anzac Day, failing to register a possession until the second quarter.
"(He's) not been at the level that we've all come to expect from Steele," McRae said.
"But he's still running incredible patterns for us.
"The ball hasn't sort of landed in his hands, in the last game if you want to just take that one for reference.
"We'd like to think Steele's got more in him in terms of getting the ball in hands, but he's a valuable player."
Injury-plagued utility Nathan Kreuger will play his first game of the season in the VFL.
