Alonso, 23, suffered the injury while working out earlier this month and is expected to miss the 2014 season.
A second-round pick out of Oregon in 2013, Alonso recorded 87 tackles and two sacks while starting every game for Buffalo last season.
The Bills also signed free agent linebacker Stevenson Sylvester, who appeared in 50 games — mostly on special teams — during the past four seasons for the Pittsburgh Steelers.
Buffalo won six games and missed the playoffs last season.
Kiko Alonso underwent successful surgery yesterday to repair his ACL. The surgery was performed by Dr. James Andrews. — Steve Caric (@Caricsportsmgt) July 18, 2014