Soria had been dealing with lower lat pain for at least two weeks, manager Ned Yost said on Wednesday, and it flared up on Tuesday. The veteran reliever got an MRI on Thursday when the team returned to Kansas City from its latest road trip.
On the season, Soria has a 3.96 ERA in 53 games, with 59 strikeouts in 50 innings.
McCarthy has made 19 relief appearances for the Royals this season. The 25-year-old has a 1.88 ERA and 11 strikeouts in 24 innings pitched.
David Adler is a reporter for MLB.