Ryu's injury occurred in Wednesday's start against the Braves, when he reached down at his lower side after a pitch in the sixth inning of the 3-2 loss.
The 27-year-old had an MRI on Friday, which showed a grade 1-2 strain. Mattingly said he is unsure of Ryu's timetable to return, and he will be reevaluated in a few days.
Said Mattingly: "This isn't an injury that they've seen a whole lot of."
Ryu, who signed a six-year, $36 million deal before last last season, is 13-6 with a 3.28 ERA in 23 starts this season after going 14-8 with a 3.00 ERA in 30 starts last season. He missed time earlier this year with left shoulder inflammation.
"Fortunately, it wasn't anything too serious," he said through a translator. "I think a short bit of rest will heal me up, and I'll be good. I definitely feel better than I did two days ago, so there's improvement, which is definitely a good sign."
Newly acquired Kevin Correia will start in Ryu's place on Tuesday against the Padres, Mattingly said.
- Chris Mascaro