In his first game back from hamstring tightness, Trbojevic limped out of Sunday's 26-12 win over Parramatta with a knee issue.
Already wearing strapping on his left knee after a collision in training on Saturday, Trbojevic battled for around 15 minutes before failing to return after halftime.
Replays appeared to show the fullback's leg become trapped under a tackle in the first half, before he sought assistance from trainers soon afterwards.
He then spent an entire set having strapping applied to his right knee, before struggling with any changes of direction and not returning after the break.
The 28-year-old left Brookvale in a knee brace after fulltime and received the verdict on scans on Monday.
Trbojevic has been ruled out for a month which denies the Sea Eagles fullback a chance to impress new NSW State of Origin coach Laurie Daley.
Lehi Hopoate's emergence as a bona fide fullback means Manly can still pose a legitimate threat without their regular No.1.
Clayton Faulalo will likely come onto the wing with Tommy Talau still at least two weeks away from returning from his hamstring injury.
But still Trbojevic's injury comes as a serious blow to the Sea Eagles, who have Melbourne, Cronulla and Penrith in the next month.
Manly's record with Trbojevic in their side goes from 38 wins in 58 matches with him since the start of 2021, compared with 16 victories in 47 games without him.
"Obviously he's very disappointed. I'm really disappointed," Seibold said.
"His preparation, he's so diligent. I think anyone who knows Turbo, he puts in so much time.
"In some ways, life's not fair because you can understand if he wasn't doing the work and doing all his reconditioning and his injury prevention and so on."
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