Manly fullback Brett Stewart will undergo knee surgery which will sideline the prolific try-scorer for between eight to 12 weeks.

In a shocking development for the reigning premiers, Stewart will go under the knife on Friday after injuring his knee in the second half of last Saturday's win over South Sydney.

Early indications after the game were that the injury wasn't serious but subsequent tests indicated it was similar to his knee injury in 2007 which required surgery.

Stewart had scored five tries in his two games this season after he was forced to sit on the sidelines for the first four rounds due to an NRL imposed suspension.

Winger Michael Robertson, who filled in for Stewart during the opening month of the premiership, is likely to take the No.1 jumper again for Saturday night's trip to face North Queensland.

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