Sylvester has a torn patella tendon in his left knee, per a source. He had been out of practice since Tuesday.
The fifth-year pro, who spent his first four seasons with the Pittsburgh Steelers, was competing for a spot on the 53-man roster. He was among the candidates to replace linebacker Nigel Bradham in the starting lineup for Week 1, when Bradham will sit a one-game suspension.
Sylvester's injury also thins out the depth at linebacker, where the Bills have already lost Kiko Alonso (torn ACL) for the season.