Cantillo was optioned to Triple-A Columbus on Tuesday.
Cantillo's demotion was expected after he served as the 27th man in Monday's doubleheader versus the Royals. He's pitched to a 7.71 ERA over 18.2 innings in the majors this year. With an off day Thursday, the Guardians may be able to get by with a four-man rotation until Alex Cobb (fingernail) is ready to return, which means Cantillo could be staying at Triple-A for a while.