The Red Sox selected Kahnle's contract from Triple-A Worcester on Thursday, Christopher Smith of MassLive.com reports.
Kahnle exercised an opt-out clause in his minor-league contract earlier this week, and the Red Sox have decided to add him to the big-league roster rather than let him reach free agency. The veteran reliever has collected a 1.40 ERA, 1.29 WHIP and 21:11 K:BB in 19.1 innings at Worcester and will fill a middle-relief role in Boston.