The move cleared the way for Cincinnati to bring up left-hander Tony Cingrani to take Cueto's turn in the rotation next week.
Cincinnati called up Henry Rodriguez before the opening game of a series Friday against the NL Central-leading Cardinals, giving them more infield depth and a switch hitter off the bench for the series. Rodriguez batted .279 with three homers and 26 RBIs for Louisville.
The Cardinals called up Kevin Siegrist and optioned right-hander Maikel Cleto to Triple-A Memphis on Thursday, giving themselves another left-handed reliever to face Cincinnati's predominantly left-handed lineup.
"We got another hitter," manager Dusty Baker said. "It's probably a temporary situation until we need that slot again. But we got a switch-hitter. Hey, it gives us another option with their two left-handers (in the bullpen)."
Villarreal was called up Wednesday to make an emergency start after the Reds put Cueto on the disabled list with a strained muscle in the back of his pitching shoulder. Villarreal gave up 10 hits and six runs in 3 2-3 innings of a 12-4 loss to Colorado.
Cingrani was called up the first time Cueto went on the disabled list this season with the same problem. The left-hander started six games and went 2-0 with a 3.27 ERA, leaving him the first choice if the Reds needed another starter.
He didn't have enough rest from his previous start to go on Wednesday, so the Reds chose Villarreal instead. Cingrani isn't scheduled to pitch again for Louisville until Monday, when he could be called up to take Cueto's place in Chicago.
Also, the Reds decided not to let outfielder Chris Heisey start a rehab assignment at Louisville on Friday as planned. Heisey has been on the disabled list since April 29 with a strained right hamstring.
Heisey's hamstring was sore after we went through drills on Wednesday, prompting the Reds to be cautious.