Koch was optioned to Triple-A Reno on Saturday.
This is a corresponding move after the Diamondbacks officially called up Anthony Banda to make his major-league debut Saturday. Koch was considered to be in the running for Saturday's start, but will instead head back to Reno after not making an appearance during his brief stint with the big-league club. Koch, 26, has an 8.47 ERA and a 1.59 WHIP in four appearances at Reno this season.