Fremantle utility Garrick Ibbotson is expected to miss six to eight weeks of AFL action after injuring his left ankle.

General manager of football operations Chris Bond said the club's medical advice was that the 22-year-old had suffered ligament damage after incurring a high-end ankle injury.

There was bad injury news for Collingwood on Monday as well, with premiership key defender Nathan Brown's season hanging in the balance, after seriously damaging a knee at training.

The Magpies said scans had confirmed the 22-year-old ruptured the anterior cruciate ligament (ACL) in his left knee during training on Monday.

Surgery options for Brown were yet to be decided.

But it would appear his only chance of a return this season would be if the backman underwent LARS surgery, involving an artificial graft.

That is an option clubs have generally been reluctant to take with young players, as the longer-term implications are considered more uncertain than using a traditional knee graft.

Port Adelaide midfielder David Rodan returned using LARS four months after rupturing his ACL last year, but needed a further operation post-season after the synthetic ligament frayed.

Brown has played 50 senior games for the Magpies since making his debut in 2008, including last year's drawn grand final and the replay, in which Collingwood broke a 20-year premiership drought.

He was rated among his team's best in both of those grand finals, given his stopping job on St Kilda skipper Nick Riewoldt.

Brown did not play in the Magpies' opening NAB Cup fixture on Saturday night, when they downed Carlton and Richmond.

His injury increases the importance of Collingwood having picked up Chris Tarrant in a trade from Fremantle last October, as he can step straight into a key defensive post.

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