Moustakas, 25, has been considered a key piece of the Royals' long-term future since the organization selected him behind Tampa Bay's David Price with the No. 2 overall pick in the 2007 first-year player draft. He hit 20 homers and drove in 73 runs in his first full season with Kansas City in 2012, but he's continued to regress over the past two years.
In 40 games this season, Moustakas was hitting .152 (19-for-125) with a .543 OPS. He was batting .109 (6-for-55) at Kauffman Stadium.
His stint was Triple-A Omaha seemed to energize him, as he returns to Kansas City after hitting .355 with a home run and five RBIs in eight games with the Storm Chasers.
Valencia, who was playing third base in Moustakas' absence, hit the DL with a sprained muscle in his left hand. He injured his hand on a swing on May 24, and missed five games before returning to the lineup Friday and going hitless in two games.
ESPN.com's Jerry Crasnick and The Associated Press contributed to this report.