Manager Bryan Price said DeSclafani felt discomfort during his last spring appearance, and an exam found the problem. It's unclear how long he might be sidelined. The Reds open the season in a little over two weeks.
"I say that with nothing but frustration for Anthony and our club,'' Price said Sunday. "I feel bad for the kid. He worked his tail off to get ready. He did everything we asked of him and more."
DeSclafani also suffered a strained left oblique before the 2016 season and was sidelined until June. The right-hander finished that season with a 9-5 record with a 3.28 ERA. He missed all of last season with a sprained elbow.
The Reds were counting on DeSclafani, Homer Bailey and left-hander Brandon Finnegan to recover from injuries and anchor a rotation that struggled without them last year. Instead, they're heading toward Opening Day with questions about two of them.
Finnegan was removed from his appearance on Sunday after only two batters because of a sore forearm. It's unclear how long he'll be limited. Finnegan was limited to four starts last season because of back and shoulder injuries.
DeSclafani won't be allowed to do any baseball activities until the injured muscle is healed, which makes it unlikely that he'll be ready to start the season.
"They want to get all the soreness out,'' Price said. "I'm sure there will be MRIs and ultrasounds to check his progress before he gets back to baseball activity."