The 25-year-old halfback suffered the injury during the Warriors’ defeat at Gold Coast last Sunday and club doctor John Mayhew says Seymour will miss at least six weeks.
Coach Ivan Cleary said the injury was a big disappointment for both Seymour and the club.
“Brett had done so much work to get himself ready,” he said.
“He played in all the trials and really impressed in those as well as the game against the Titans. We’re all disappointed for him.”
Seymour has been replaced by former Melbourne player James Maloney, who starred in Saturday’s 30-16 win over Cronulla.
Joel Moon filled in the vacant five-eighth role by coming off the bench.