Young midfielder Leigh Adams injured his knee while playing for the Kangaroos' VFL affiliate Werribee against Bendigo on Sunday.
Adams, who had been elevated from the rookie list last year, had initially thought he would only miss a handful of weeks after suffering the injury.
But scan results have indicated he has torn his anterior cruciate ligament off the bone and will require a full knee reconstruction.
Werribee coach Simon Atkins said Adams injury looked "inconspicuous".
"He took the football, went to side-step a bloke and then he couldn't, and another bloke collided with him. But it looked like he'd done it (the knee injury) before the collision," Atkins said.
Adams joins rookie Nathan Grima on the sidelines with season-ending knee injuries.
But in better news for the Kangaroos, defender Jess Sinclair came through his first match back from a thumb injury in the VFL unscathed and a week off has seen ruckman David Hale improve to be available for selection against Collingwood on Saturday night at the MCG.
