Sironen will meet with surgeons on Tuesday, where he is likely to be told he will need surgery after getting caught awkwardly attempting a tackle in Sunday's win over Cronulla.
His absence will take some serious air out of a flying Sea Eagles side that have surged into the top four for the first time since 2014.
The former Wests Tigers forward is considered one of the most improved players in the competition this year following his move to the northern beaches over the summer.
He had started in every single game under coach Trent Barrett this season, and scored two tries against the Sharks in what was arguably his best performance of the year.
"He's been one of our best players this year," teammate Dylan Walker said on Tuesday.
"He was really starting to find form, it's a bit upsetting and a bit sad for him because he's had so many injuries over the years. Hopefully it's not too bad and we can get him back soon."
The Sea Eagles will aim to continue an impressive record away from Lottoland this season when they host the Warriors in Perth on Saturday.
Their only loss on the road came when they took a home game to Suncorp Stadium in round 10, only to lose to the ground's tenants Brisbane.
Walker credited his team's strong away form to a greater team bond compared to last year, however he believes it's a growing trend across the league.
"Everyone here are real close together, it could be the little things of going out to golf, coffee or brekky. We definitely like each other's company, and it's just a close-knit group," he said.
"There's a lot of teams doing good away from home. You only look at State of Origin, when they've both won away. It might be something new happening.
"...Playing away is something we like doing and hopefully we can continue to do so."
Photo by: Paul Barkley/NRL Photos (Sea Eagles second-rower Curtis Sironen).