Cahill, acquired by the Royals in a multiplayer trade with the San Diego Padres, has struggled in three starts with Kansas City. He has a 8.18 ERA in three starts, allowing 10 earned runs in 11 innings. He also has walked nine batters while striking out just six and has allowed 18 hits, including four home runs.
The Royals recalled right-handed reliever Kevin McCarthy to take Cahill's spot on the roster. McCarthy, 25, has pitched in 21 innings for the Royals this season, posting a 2.14 ERA with a 1-0 record. He was 1-1 with a 3.13 ERA in 31⅔ innings at Triple-A Omaha this season.
Cahill, 29, pitched on Wednesday night against the St. Louis Cardinals but lasted just 2⅓ innings, allowing three runs on four hits and walking five.
He also went on the disabled list earlier this year with a shoulder strain in May when he was pitching for the Padres. He was much more successful in San Diego this season, going 4-3 with a 3.69 ERA in 11 starts. He struck out 72 while walking 61.
The Royals (57-56) enter Thursday night's game against the Cardinals one game behind the Seattle Mariners for the second wild-card spot in the American League.