Osuna was sidelined on Aug. 2, and this roster move guarantees he won't return until mid-September.
The 25-year-old was removed from an outing against the Los Angeles Angels on Aug. 1 and sent for an MRI. After Osuna was placed on the IL with right elbow soreness, sources told Mark Berman of Fox the initial diagnosis indicated a need for Tommy John surgery. However, no official decision has been made.
The 2017 All-Star led the majors in games finished (56) and the American League in saves (38) last year while authoring a 2.63 ERA and 3.21 FIP over 65 innings.