SCANS have revealed Greater Western Sydney key back Lachie Keeffe re-ruptured his right ACL in the final minute of the Giants' nail-biting win over Essendon.
He is likely to have surgery later this week, which will rule him out for the rest of the season.
Keeffe previously injured the ligament in 2012 when playing for Collingwood.
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The 31-year-old suffered the injury after landing awkwardly in a marking contest.
He signed a one-year contract extension just last week to tide him over at the Giants till the end of 2022.
"It's obviously devastating news for Lachie and we're throwing our arms around him," GWS head of medical Cullan Ball said.
"He was injured very late in the game and assessed following the match.
"Further assessment on Sunday indicated there may be an issue with the ACL and subsequent scans revealed Lachie will require a knee reconstruction.
"He'll head to Melbourne on Wednesday to see Dr Julian Feller and we expect him to undergo surgery later this week."
Fellow Giants Jesse Hogan (calf), Matt Buntine (concussion) and Tom Hutchesson (hamstring) are all tests for Saturday night's clash with Richmond, while hamstrung pair Nick Haynes and Sam Reid are one week away.