LOS ANGELES — Dodgers ace Clayton Kershaw went on the injured list because of a back issue hours before he was scheduled to start Thursday night for Los Angeles against the San Francisco Giants on opening day of the shortened season.

Manager Dave Roberts said Kershaw's back stiffened up in a weight room workout two days ago. He's been receiving treatment ever since, but the training staff advised against the 32-year-old three-time NL Cy Young Award winner making his ninth opening day start.

"He's as prepared a player as I've ever been around and starting an opening day means a lot to him," Roberts said. "For him to not get to experience it for the ninth time, I just feel for him."

Roberts said that while there is no set timetable for Kershaw's return, he could be available during the Dodgers' series at Arizona from July 30-Aug. 2.

Kershaw was replaced by 22-year-old right-hander Dustin May, who will become the first Dodgers rookie to start on opening day since Fernando Valenzuela in 1981.

"He's very unflappable. He's a very confident young man," Roberts said. "He's got obviously really good stuff, so when a change of plans happens, Dustin adjusts very well."

Kershaw has been bothered by back problems in the past. He was on the IL in 2016, 2017 and 2018, each time with various issues.

The left-hander had a 3.30 ERA last season, his highest since his rookie year in 2008.

May was recalled Thursday after being optioned to the team's remote training location. He had been set to throw a simulated game a day earlier, but it didn't happen.

May had a 3.63 ERA in 14 appearances, including four starts, last season with the team.