Collingwood will be without No.1 ruckman Josh Fraser for at least two more AFL games after the Magpies decided he should undergo knee surgery before the finals.

Fraser injured his knee against the Western Bulldogs in round 14 and sat out last Saturday night's defeat to Hawthorn.

He has been ruled out of Friday night's blockbuster against Carlton, where the Magpies will aim to keep hold of their spot in the top four, and the round 18 clash against the Brisbane Lions.

Coach Mick Malthouse is hopeful Fraser can return against Adelaide in round 19.

Fraser's knee troubles date back to last year's state of origin game, where he was hurt playing for Victoria against the Dream Team and spent five games on the sidelines.

While his injury is a major blow, Malthouse is hopeful Fraser's absence will provide Cameron Wood with the opportunity to spark his career.

Wood has had little influence in the 19 games he has played since he was traded from the Lions in 2007, but Malthouse said the chance to play as first-choice ruck could bring the best out of the 22-year-old.

"When you have responsibility sometimes that brings the best out of you and sometimes the worst out of you," Malthouse said on Thursday.

"So this is a great test for him, but we're reasonably confident.

"He has played some very good football against some pretty good opposition this year, but we've always elected to see if we can get about 75 per cent game time out of Josh in the ruck.

"That's not real kind to a second ruckman."

A knee problem will also keep defender Nathan Brown out of the game, but fullback Simon Prestigiacomo proved at training he had overcome a knock to the ribs and was ready for a duel against Carlton full-forward Brendan Fevola.

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