Moustakas left Sunday's game against the Minnesota Twins after five innings. He's missed four games.
Moustakas, hitting .301 in 75 games, is one of four players competing for the final AL All-Star spot. Voting results will be announced shortly before game time.
To make room on the roster, infielder Cheslor Cuthbert, who had a hit in all four games he played with the Royals, has been optioned to Triple-A Omaha. The Royals went 4-0 with Cuthbert starting at third base.