Bonilla, who was claimed off of waivers from the Pirates at the start of Spring Training on Feb. 13, was 2-0 with a 5.56 ERA in two starts for Louisville.
"He's stretched out, he's been a starter in Triple-A," Reds manager Bryan Price said. "We're flipping pitchers in large part because we need some depth in relief pitching. We haven't really gotten into our groove yet with our rotation where we're getting that consistent six-plus innings per game."
Romano threw 82 pitches and lasted three innings during Sunday's 4-2 loss to the Brewers. He allowed three runs (two earned) and three hits with four walks and two strikeouts.
In the past week, the Reds have called up three relievers to fortify the bullpen in Bonilla, Tim Adleman and Barrett Astin. Adleman is now scheduled to start on Friday vs. the Cubs while Astin was sent back to Louisville.
Bonilla last pitched in the big leagues in 2014 for the Rangers and was 3-0 with 3.05 ERA in five games, including three starts.
"It's something really big for me," Bonilla said via translator Julio Morillo. "Right now, I'm just going to be focused like I was in the Minor Leagues. Hopefully, I can stay here for the rest of the season. My arm feels really good right now, so we'll see what happens."
Mark Sheldon has covered the Reds for MLB.com since 2006, and previously covered the Twins from 2001-05. Follow him on Twitter @m_sheldon and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.