Jonathon Patton and GWS are bracing for bad news on Friday, with scans expected to confirm the AFL star needs a third knee reconstruction.
Patton hurt his left knee at training on Thursday and the Giants fear the worst.
If the ruck-forward has suffered an anterior cruciate ligament rupture, he will need a reconstruction and that would put him out of action for about 12 months.
More immediately, it is a massive blow to the Giants' finals hopes, which were on the rise after four-straight wins.
GWS already have left Patton out of their squad for Sunday's Perth match against West Coast.
Giants veteran Brett Deledio and club football boss Wayne Campbell said it does not look good for Patton.
"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."
The former No.1 draft pick needed reconstructions in 2013 and 2014 after ACL injuries in his right knee.
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 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.
