Callahan will undergo shoulder surgery, Betsy Helfand of the Las Vegas Review-Journal reports.
Callahan, who has been on the minor-league disabled list since April 22 with a shoulder injury, will miss the remainder of the season after going under the knife. Prior to suffering the injury, the 23-year-old compiled a 9.72 ERA and 9:4 K:BB across 8.1 innings of relief.