Rainey was optioned to Triple-A Louisville on Sunday.
Rainey's stint with Cincinnati lasted only three days after he was summoned from Louisville to provide an extra arm out of the bullpen while Amir Garrett was on the bereavement list. The hard-throwing right-hander should recapture a late-inning role at Louisville, where he had struck out 17 batters in 9.2 scoreless frames prior to his promotion.