Colon had been hitting .228/.295/.268 with 10 RBIs in 47 games, including 33 starts, with the Royals this season.
The switch-hitting Nava, who was acquired from the Angels in a trade on Aug. 30, gives Royals manager Ned Yost a left-handed bat off the bench. Nava hit .235/.309/.303 with a homer and 13 RBIs with the Angels before he was designated for assignment on July 30.
"If we pinch-run for [Kendrys] Morales at the DH spot and that spot comes back around, we feel like we're covered with Nava from the left side, Hunter Dozier and Colon from the right side," Yost said before Tuesday's game against the Twins. "It's just depth."
McCarthy, 24, earned his first Major League callup. He went 2-4 with a 2.97 ERA at Omaha after being moved up from Double-A Northwest Arkansas on June 16.
"We just needed another arm," Yost said. "Our guys have been really used a lot in the last 10 days and another arm always helps."
Do-Hyoung Park is a reporter for MLB.com based in Minneapolis. He covered the Royals on Tuesday. This story was not subject to the approval of Major League Baseball or its clubs.