"He's a guy that we signed for depth," manager Ned Yost said. "He's a guy that's had success in the big leagues. ... It's another experienced arm."
Saunders was an American League All-Star for the Angels in 2008, when he finished 17-7. He has a career record of 89-86 record with 229 Major League starts for the Angels, Diamondbacks, Orioles, Mariners and Rangers.
One of his most memorable victories came on May 9, 2009, when he and the Royals' Zack Greinke had a complete-game duel at Anaheim. Saunders won the game for the Angels, 1-0, with a five-hitter. Greinke won the AL Cy Young Award that year.
The Royals' pitching rotation has been solid this season and there's no need for a starting pitcher.
"How quick does that change?" Yost said. "So, you've got make sure you've got enough experienced arms down there."
The Royals also have former big league starter Clayton Mortensen at Omaha with a 4-0 record and 1.64 ERA.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.