PHILADELPHIA — It wasn’t until late in Matt Cain’s ninth big league season that he finally became acquainted with the disabled list. Now the longtime San Francisco Giants workhorse can’t seem to avoid it.

Cain on Monday was put on the disabled list with irritation in his right elbow, giving him three DL trips this season and four since last August 23. This injury is easily the most serious. It’s the first that involves his pitching arm. Cain said he was relieved that an MRI came back clean and showed no ligament damage, but he was unable to give a timetable for his return.

“We’ll let it calm down and get back to throwing, and we’ll go from there,” he said.

And when will he throw again?

“I can’t tell you when he’ll throw,” Giants manager Bruce Bochy said. “I don’t think we know. This thing can go down in a day or two, or it could linger. I don’t know how it will affect him the rest of the season.”

That’s a scary thought for an organization already looking at numerous leaks to plug as the July 31 trade deadline approaches. For the time being, Yusmeiro Petit will take Cain’s rotation spot, and George Kontos — recalled from Triple-A Fresno — will fill out the bullpen.

The Giants are hopeful the arrangement doesn’t last long, but they don’t have any definitive answers. The club discovered in 2010 that Cain has bone chips in his right elbow, and it’s said to be a significant amount, hardly surprising since Cain has pitched more than 1,800 big league innings and carried a heavy load in two postseasons. Any cleanup procedure would likely end Cain’s season, and he isn’t keen on going under the knife in the offseason, either.

“You try to stay away from having surgery,” he said. “Everybody has got stuff going on (in the elbow). If you can stay away from surgery, you do. That’s the last option. There’s no reason to need that step right now. We’ve got time to let it heal and go that way with it.”

While Bochy has said Cain’s elbow has been “cranky” since spring training, Cain refused to connect his subpar results to the inflammation. He is 2-7 with a 4.18 ERA in 15 starts and is striking out just seven batters per nine innings pitched, a career low. Cain has thrown at least 184 innings in every full big league season, but he’s at just 901/3 thanks to a finger laceration, hamstring strain and this inflammation.

“When you see guys that are able to run out there every fifth day and position players that play all the time, that’s something you take pride in,” he said. “Some of it is luck, some of it you can’t control.”

The Giants don’t have much control right now. They’ll settle for luck.