Making his return from a dislocated right shoulder suffered six weeks earlier, Olakau'atu suffered the same injury in the Sea Eagles' 42-4 loss to Canterbury on Sunday.
Manly had indicated a season-ending surgery was all but certain, before confirming on Monday the second-rower won't return until January.
It means the NSW State of Origin representative will also be unavailable for Tonga in the end-of-season Pacific Championships, and miss part of next year's pre-season.
Olakau'atu was hurt when make a seemingly innocuous tackle on Sunday, after suffering the initial injury while attempting to score a try against Gold Coast last month.
"When you hurt your shoulder, I think there's around about a 90 per cent risk you'll do it again," Manly coach Anthony Seibold said on Sunday.
"Obviously with a player of his quality and standing in our team, we wanted to pursue that risk. We didn't want to worry about next year."
Olakau'atu's injury means he has played his last game alongside departing captain Daly Cherry-Evans, with the duo having formed a dangerous combination on Manly's right edge.
Corey Waddell will likely come into the starting side for the crunch match against the Sydney Roosters on Saturday at Brookvale, with both teams fighting to play finals.
Manly have extremely limited back-row depth, and opted to play winger Tommy Talau on the bench during Olakau'atu's last absence.
Alternatively, Michael Chee Kam could be considered as an option, after not playing NRL at all this year.
The Sea Eagles are already without forwards Josh Aloiai (shoulder) and Nathan Brown (bicep) for the rest of the season, while Taniela Paseka is fighting to return from an achilles tendon rupture in time for for finals.
Meanwhile Canterbury are also counting the cost of Sunday's win, with fullback Connor Tracey suffering an abdominal tear.
Tracey was injured in the Bulldogs' win, but managed to see out the game.
Canterbury's reserve fullback Blake Taaffe is also sidelined with a hamstring injury, while Blake Wilson also has experience playing No.1.
Alternatively, the Bulldogs could shift Jacob Kiraz or Stephen Crichton to fullback on Sunday against Wests Tigers, as part of a backline reshuffle.
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