GWS defender Lachie Keeffe tore his ACL during his side’s win over Essendon.
The key defender suffered the injury in the final minute of the game, according to the Giants.
GWS held on to beat Essendon by two points in a high scoring, back and forth contest.
The 31-year-old is out of contract at the end of the season, but had played all eight games so far this season and impressed as a key defender.
He has played 79 career games for the Giants and Collingwood.