After a fortnight of spotlight on Seibold's job and the Sea Eagles' form, Manly were far from their best at Brookvale Oval but still put themselves back in the top eight.
Their plan to give Tom Trbojevic some confidence playing at centre showed some signs of working, with the regular No.1 having a role in two tries.
But his replacement at fullback Lehi Hopoate had one of his toughest outings of the season, putting down two high balls and getting beaten to another.
Still, it was enough for the Sea Eagles to consign the Tigers to their sixth straight loss, who desperately missed Jarome Luai in attack after he was ruled out with a boil.
Adding to the Tigers' concerns was a concussion for Api Koroisau, that has the potential to stifle any chance of Tallyn Da Silva leaving before the June 30 deadline.
After back-to-back losses to Newcastle and Gold Coast before last week's bye, Manly just needed a win no matter how ugly it was.
And for large periods of the first half on Friday night, it was anything but pretty for both teams through a scoreless opening 39 minutes.
Trbojevic was whacked by Jeral Skelton the only time he received it in a good area during that time, with Manly regularly going short rather than out the back.
But when the Sea Eagles finally utilised the fullback-turned-centre again, the game started to turn.
The only first-half try came through the 28-year-old, when Trbojevic got the ball 10 metres out and put Jason Saab over to score.
Trbojevic was at it again to start the second half, going from right centre to the left side of the field to act as second receiver.
That gave the Sea Eagles an extra number, with Trbojevic holding the ball up for Lehi Hopoate to send Reuben Garrick over with a cut-out ball.
Another Manly try came down that side after Trbojevic had again created numbers early in the set, before Luke Brooks and Daly Cherry-Evans helped Tolu Koula score.
Second-rowers Corey Waddell and Ben Trbojevic also went over in the second half, one from a Cherry-Evans short ball and another from the halfback's kick.
There were still some questions over Tom Trbojevic's defence at centre, with the visitors' first try coming when Adam Douiehi put Alex Seyfarth past him and into a gaping hole.
While the Tigers are clearly an improved outfit, they have now lost six in a row and will likely face the Sydney Roosters next week without Koroisau and Luai.
Da Silva scored late out of dummy-half but looks likely to leave the club by year's end, while Latu Fainu lacked spark without Luai next to him.
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