According to Yahoo Sports, the starter was placed on injured reserve on Wednesday due to a ruptured biceps. He will likely be replaced by second-year linebacker Philip Wheeler and undrafted rookie Ramon Humber.
Rookie Cody Glenn will be promoted to the active roster.
Hagler, currently fourth on the team with 26 tackles, is in the midst of his fourth year with Indianapolis. In his career, he has 114 tackles and two forced fumbles in 37 games.