General manager Doug Whaley provided the update in a statement released by the team on Wednesday, a day after Alonso was hurt while working out in Oregon. Whaley said Alonso will require surgery to repair a torn ACL, and ''will likely miss the 2014 season.''
The Bills did not provide any details as to how Alonso was hurt.
The injury is a significant blow to the Bills before they even open training camp in suburban Rochester on July 20.
Alonso was an NFL defensive rookie of the year contender last season after being drafted in the second round out of Oregon. He led Buffalo with 159 tackles, two fumble recoveries and tied for the team lead with four interceptions while barely missing a snap in playing all 16 games.
After playing in the middle last season, Alonso was moving to the outside linebacker spot as part of the Bills' shift to a 4-3 defensive scheme under new coordinator Jim Schwartz. The position switch was also sparked in part by the Bills' signing of run-stopping specialist Brandon Spikes, who is better suited to play in the middle.
Alonso had hip surgery this offseason, which limited him in participating in spring practices. But he was expected to be ready for the start of camp.
''We feel badly for Kiko, because he has worked very hard this offseason in preparation for the upcoming season,'' Whaley said. ''But we look forward to his return with his style of play that made him one of the league's outstanding rookies in 2013.''
The Bills do have some depth behind Alonso.
Manny Lawson, an eight-year NFL veteran, has experience playing the position. The Bills also selected linebacker Preston Brown in the third round of the draft in April.
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