The outfielder-designated hitter was signed three years ago to a $36 million, three-year contract that made him the Royals' highest paid player per year in team history. He was leading the Royals with 62 RBIs and 16 home runs, and he is owed approximately $3.93 million from his $12 million salary this season.
Kansas City is Guillen's ninth major league stop. The Royals have 10 days to trade or release him.
The Royals also selected the contract of right-hander pitcher Philip Humber from Triple-A Omaha.