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]."
Tom Singer is a reporter for MLB.com and writes an MLBlog Change for a Nickel. He can also be found on Twitter @Tom_Singer. This story was not subject to the approval of Major League Baseball or its clubs.