Castro was optioned to Triple-A Iowa on Tuesday, Maddie Lee of the Chicago Sun-Times reports.
Castro will head back to Iowa after getting called up Aug. 3. While in the big leagues, the 23-year-old allowed seven runs on seven hits and five walks while striking out four over 7.2 innings in six appearances. Javier Assad was called up from Triple-A to replace Castro on the roster and start Game 1 of Tuesday's doubleheader with the Cardinals.