Dennis suffered torn ligaments in his left knee during a scrimmage after playing sparingly this season. He started every game in 2003. The Los Angeles Times reported he is expected to have surgery within a month.
There is a chance the injury could force Dennis to sit out next season.
Dennis was suspended for nearly a month this season for violating team rules.
"He's had such a hard year -- my heart goes out to Hershel," Coach Pete Carroll told the newspaper.