Ramirez started on Saturday against the Giants at Great American Ball Park.
A temporary replacement in the starting rotation for Johnny Cueto, who's nursing a strained right elbow, Ramirez went 4-5 with a 3.08 ERA in 19 appearances, including 15 starts, for Louisville.
Prior to his promotion to Triple-A, Ramirez went 2-3 with a 4.70 ERA in 11 appearances at Double-A Chattanooga. He last pitched on Monday, allowing just five hits in six shutout innings in the Bats' 6-3 victory over Columbus.
Reds catcher Ryan Hanigan, who caught Ramirez on a few occasions in the Minors, will start behind the plate on Saturday.
"Hanigan knows him pretty well," said Reds manager Dusty Baker. "That's important. I remember [from Spring Training] that [Ramirez] has a good arm. They said not to judge what I was seeing then. I was told he's a better second-half pitcher."
Ramirez, who will wear uniform No. 58, will become the eighth Reds player to make his Major League debut this season, and it's the 39th time that they have used a rookie pitcher, following Cueto, Homer Bailey and Daryl Thompson.
For Rosales, it was the third transaction involving him in the past two weeks, including twice being optioned to Triple-A. Rosales' latest stint in the Minors is expected to be brief.
"He'll play at Louisville and try to help them win [in the playoffs]," said Baker. "He'll be back once the playoffs are over."
The condition of Cueto, meanwhile, is being closely monitored. He last pitched Sunday at Colorado, but exited after just three innings with right elbow soreness.
Cueto, who is 8-12 with a 4.65 ERA in 27 starts, threw long-toss on Saturday and was expected to have a couple of bullpen sessions before being evaluated again.
"He's feeling good, and he's chomping at the bit to pitch," said Baker, who quickly deflected a suggestion that Cueto be shut down for the remainder of the season.
"If you're OK to pitch, then pitch," Baker said. "It all depends on how he feels. Maybe this rest will do him good."
Jeff Wallner is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.