Butler, 28, was an All-Star in 2012 when he hit .313 with 29 homers and 107 RBI, but his numbers have declined since then. In 151 games this season, Butler hit .271 with nine home runs and 66 RBI. He hit .262/.327/.333 this postseason, with eight RBI and three doubles. He was 5-for-15 with a double and three RBI against the San Francisco Giants in the World Series.
Butler will be owed a $1 million buyout.
Aoki, who will be 33 in January, was acquired in a trade from the Milwaukee Brewers for left-hander Will Smith last December and hit .285 in 132 games with one homer, 43 RBI and 17 stolen bases. Aoki struggled in the postseason, hitting .195 with three RBI in 48 plate appearances. He was just 1-for-14 in the World Series.
The Royals already began preparing for the 2015 season on Thursday by trading pitcher Liam Hendriks to the Toronto Blue Jays.