Greater Western Sydney star Jonathon Patton could need another knee reconstruction, meaning a cruel blow to his AFL career and the Giants' finals hopes.
Patton hurt his left knee in a marking contest during training on Thursday and scans will determine the extent of the injury.
But GWS fear it is another anterior cruciate ligament (ACL) rupture.
"There's a little ray of hope for him, that it isn't as bad as we think," GWS veteran Brett Deledio said on ABC Radio.
"But I think our fears will ring true tomorrow.
"It's not ideal for big Jon. Our thoughts are with him and fingers crossed."
Giants football boss Wayne Campbell admitted things didn't "look positive" for Patton.
The former No.1 draft pick needed reconstructions in 2013 and 2014 after ACL injuries in his right knee.
If the 25-year-old requires another reconstruction it's likely to sideline him until the middle of next season.
More immediately, the ruck-forward's injury is devastating for GWS and their finals hopes.
They have endured a rotten run of injuries this year, but had rallied lately with four straight wins, returning to the top eight.
The news is better for Deledio and Toby Greene, who will miss Sunday's clash with West Coast in Perth but are upbeat they could be cleared to return against Richmond the following round.
Deledio (calf) and Greene (foot) have both missed two months of football.
"I've got a big session on Saturday to tick off then next week I'll be putting my hand up for sure," Deledio said.
"We haven't been blessed on the injury front this year. We've had three boys go down with ACLs."
Patton's injury means the Giants will be without their top four goalkickers from last season.
Apart from Patton and Green, Jeremy Cameron has another four games to serve of his suspension and Steve Johnson has retired.
The Giants are in the midst of a season-defining block of games against other top-eight teams.
They beat Hawthorn last week and after the Eagles they host reigning premiers Richmond, before another road trip against Port Adelaide.
West Coast's attack also is well below strength, with Josh Kennedy, Jack Darling and Mark LeCras all injured.
Liam Ryan is also unavailable after he crashed his car into a tree during the week.
Patton is a foundation GWS player and the 2011 No.1 draft pick has played 89 senior games.
He also needed knee surgery to fix a patella tendon problem in his left knee before the 2012 season.
Patton hurt his left knee in a ruck contest during last weekend's win over Hawthorn, but played out the match and kicked the match-sealing goal.
