Brett Stewart's annus horribilis continued on Tuesday, but Manly are adamant the loss of their star fullback for up to three months will not derail their NRL premiership defence.

Just days after producing their best performance of the season, the Sea Eagles were dealt a body blow with Stewart to undergo surgery on Friday after damaging knee ligaments in last Saturday's win over South Sydney.

The injury, which coach Des Hasler described as "shocking luck for Brett", comes with a rehabilitation period of 8-12 weeks, the worst-case scenario keeping the prolific tryscorer on the sidelines until Manly's round 19 encounter against Cronulla.

Stewart's return after missing the first four games through suspension had sparked a revival at Brookvale, the former NSW and Test custodian scoring five tries in his two appearances before injury struck.

"It's disappointing after two fantastic games back," Sea Eagles chief executive Grant Mayer said.

"He actually started hobbling just after halftime and he played through it - when it started to cool down it started to hurt more.

"We put him through some scans on Monday and then we got the results last night. We knew there were some major problems then and then this morning the worst was confirmed, 12 weeks."

Stewart's year started on a high with the 24-year-old playing a starring role in Manly's World Club Challenge win.

He then joined brother and Manly backrower Glenn in launching the NRL season which featured an advertising campaign in which Brett starred alongside Melbourne sensation Greg Inglis.

Things went downhill from that moment, however, with Stewart charged with the sexual assault of a teenage girl following the club's alcohol-fuelled season launch.

His sexual assault charge continues at Manly Local Court on May 26.

He was then suspended by the NRL for the opening four matches of the season under the league's code of conduct, with Manly going win-less with their try-scoring whiz sitting in the stands.

Mayer said the experience of playing without Stewart earlier in the season and the confidence gained following his return would ensure there would not be a repeat of the club's barren streak from the opening four rounds.

"We've won football games without Brett previously, there's no doubt at the start of this season it was certainly a unique position for us as a club," Mayer said.

"I think we had Brett back for two games, the confidence within the club and the team lifted, and I think we realised how good a football team we are and can be.

"I don't think that we're going to fall back into the type of form we showed in the first four weeks."

Stewart's influence in his two games resulted in him being been talked up as a chance of making the Australian squad for the May 8 Test against New Zealand.

He was included in NSW's preliminary 40-man squad for this year's State of Origin series announced on Tuesday, though he is unlikely to play any part in the series with game three to be held the week before he is due to return.

"When the selectors picked the team, obviously they weren't aware of the extent of his injuries," NSW general manager Geoff Carr said.

"... obviously if it is the worse-case scenario, he won't be available."

Bookmakers failed to share Mayer's optimistic outlook, with the Sea Eagles blowing out from $7 second premiership favourites to the fifth line of betting at $9.

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