The Pirates placed left-hander Wandy Rodriguez on the 15-day disabled list Monday with right knee inflammation.

It’s unclear how or when Rodriguez injured the knee. He pitched Saturday and was scheduled to pitch Thursday.

“Wandy is not sure how he hurt his knee,” Pirates manager Clint Hurdle said. “He’d gone to the training room after his last start and complained of some right knee discomfort.”

Rodriguez was scheduled to have an MRI Monday.

“He’d felt it a couple times in his last start, didn’t think anything of it,” Hurdle said. “But I guess they extracted a large volume of fluid from his knee. … That’s all I’ve got.”

Hurdle did not name a starter to replace Rodriguez in the rotation, but said an answer is likely to come in-house.

“We’ve got a couple different guys in-house. We’ll see who lines up,” said Hurdle.

Rodriguez, who spent most of 2013 on the DL for a forearm injury, has struggled out of the gate this season.

He has allowed 26 hits, 17 runs and seven homers in 20 innings pitched over the course of four starts.

His ERA soared to 7.65 and his fastball velocity dropped a nearly 1 mph from 2013.

Jared Hughes was called up Monday to replace Rodriguez and offer bullpen help. Hughes recorded a win and three saves with Class AAA Indianapolis in six innings over five appearances.