Roberts said on Thursday that Ryu tore the muscle off his bone.
"It's a pretty dramatic injury,'' the manager told reporters before the Dodgers and Diamondbacks wrapped up a four-game series with an afternoon game.
Ryu left the game in the bottom of the second inning after recording an out, following a visit to the mound by Roberts and a trainer to check on him. He lasted 1 innings and struck out two with a walk, allowing one hit.
Ryu is 3-0 with a 2.12 ERA this season.
He missed a month with a similar injury in 2016.
Walker Buehler, the 23-year-old top prospect who has had two strong starts for the Dodgers this season, is in line to take Ryu's rotation spot.
"It seems like the logical choice,'' Roberts said.
Buehler is expected to be recalled from the minors again to start Friday night's game against San Diego in Monterrey, Mexico, then be worked into the rotation from there. He is 1-0 with a 1.80 ERA in two starts for the Dodgers with 11 strikeouts in 10 innings pitched.
The Associated Press contributed to this report.