Muller was optioned to Triple-A Las Vegas on Sunday.
Muller was recalled from Las Vegas on Saturday for his first big-league start in five weeks, and he'll return to the affiliate after he gave up three earned runs over five innings. The left-hander has a 7.79 ERA over 11 starts in the majors and a 6.59 ERA in six starts at Triple-A this season, so he shouldn't be on the fantasy radar even if he rejoins Oakland's rotation later this summer.