During his exam with team medical director Dr. Tim Kremchek, an MRI taken on Bailey showed right shoulder inflammation. There was no structural damage.
"It could've been worse news, that's how you've got to look at it," Reds manager Dusty Baker said.
Del Rosario, who will wear No. 58 and make his big league debut when he appears, is a side-armed reliever that impressed in Spring Training. In 20 appearances for Louisville, he was 1-1 with a 1.46 ERA. He had four walks and 17 strikeouts over 24 2/3 innings.
The Reds plan to call up a starter to take Bailey's turn in the rotation on Friday, when they open a series vs. the Astros. It will either be Sam LeCure, Matt Maloney or Travis Wood from Louisville, general manager Walt Jocketty confirmed.
"We'll probably make that decision the next couple of days," Jocketty said.
It will definitely not be Aroldis Chapman, who missed his start for Louisville on Monday because of a blister on his left middle finger.
Bailey, 24, lasted only 2 1/3 innings in his start on Sunday vs. the Indians and was lifted after his 46th pitch because he experienced tightness in his right shoulder. In nine starts this season, the right-hander is 1-2 with a 5.51 ERA.
The Reds were inclined to make a move because starter Johnny Cueto has been dealing with a blister. There was concern that if either or both had another shortened outing this week, it would tax a tired bullpen further.
"The recommendation for him right now is to not doing anything for a week," Jocketty said of Bailey. "He could pitch with it but it's going to get worse, not better. The best thing to do is shut it down, let the inflammation subside and then he can start throwing again. It's tough because I know he's competitive and he wants to be out there. Sometimes, it's better to take a step backwards so we can go forward."
Kremchek offered to give Bailey a cortisone shot to get rid of the inflammation but the pitcher refused.
"I'm not a big advocate of cortisone shots," Bailey said.
Bailey has long been one of the Reds' coveted young pitchers and this was his first injury involving his right arm.
"Kremchek said this was not from overwork or overuse," Jocketty said. "He may have tried to use too much effort on a pitch or two and it caused the problem and he kept trying to pitch with it. He didn't throw too many pitches."