PITTSBURGH -- The Pirates placed Wandy Rodriguez on the 15-day disabled list on Monday, and they filled his spot on the staff by recalling right-hander Jared Hughes from Triple-A Indianapolis.

Rodriguez has struggled early this season, posting a 7.65 ERA in four winless starts. But it is not his left arm that sent him to the DL, but his right knee. The club said the 35-year-old is suffering from inflammation in the joint.

Rodriguez had an MRI and a visit to the doctor, who drained fluid from the knee.

"We had a long conversation, and he's not sure how he hurt it," manager Clint Hurdle said. "He went into the trainer's room after his last start [on Saturday], complaining of discomfort. He felt it a couple of times in the start and didn't think anything of it."

Hughes was unscored upon in five appearances with the Triple-A Indians. In six innings, he gave up two hits and two walks, with seven strikeouts.

"The sinker has been in a good place," Hughes said. "It has to be; that's the bread and butter."

The 28-year-old reliever adds depth after the Pirates had emptied their bullpen in Sunday's 14-inning loss to Milwaukee. However, Hughes' tenure may extend beyond that short-term quandary.

Hurdle indicated Rodriguez's spot in the rotation would be filled either by long relievers Jeanmar Gomez or Stolmy Pimentel.

"We'll have a pitcher to take his spot," Hurdle said. "We've got a couple of different guys in-house, and we'll just see who lines up best [to make that start on Thursday in the finale of the four-game series against Cincinnati]."