Farrell was optioned to Triple-A Louisville on Sunday, Zach Buchanan of the Cincinnati Enquirer reports.
Farrell's demotion creates room on the 25-man roster for Tyler Mahle, who was promoted from Triple-A Louisville to start Sunday's game against the Pirates. The 26-year-old Farrell was with the Reds for less than a week, making his long appearance in long relief Wednesday against the Cubs. He could rejoin the big club when rosters expand in September.