The Royals made the move Saturday before playing Boston.
Cain was hurt while taking batting practice Friday. He is hitting .261 with 43 RBIs in 96 games.
The Royals also designated for assignment left-handed pitcher Francisley Bueno, who picked up the win Friday over Boston by throwing 2 1-3 scoreless innings in relief.
Kansas City recalled left-handers Will Smith and Donnie Joseph from Triple-A Omaha. This is Smith's fifth stint with the Royals this season, going 1-1 with a 3.26 ERA in seven outings. Joseph made two appearances with the Royals in July.