A.J. Burnett dealing with torn calf

PITTSBURGH -- A.J. Burnett is wearing a walking boot over his torn right calf and isn't sure when he'll be ready to return.

The Pirates right-hander was injured while working out Wednesday and was placed on the 15-day disabled list a day later. The 36-year-old called it a "freak" accident, saying he was finishing up a series of sprints when something went wrong.

"I felt something and it cramped up after that," Burnett said. "It was just one of those things."

Burnett is 4-6 this season with a 4.00 ERA and is third in the National League with 99 strikeouts. He threw a season-high 112 pitches in a win over the Chicago Cubs last Saturday but said that outing and the injury are not related. He waited 24 hours before telling Pittsburgh manager Clint Hurdle, hoping the pain would go away. It only got worse.

"I can barely put any pressure on it," Burnett said. "I'm treating it all day. All day yesterday, all day today. Just got to stay off it."

Rookie Brandon Cumpton will his major league debut when he starts in place of Burnett on Saturday against Clayton Kershaw and the Los Angeles Dodgers. Cumpton is 4-4 with a 3.41 ERA for Triple-A Indianapolis this season.

Burnett is the fourth member of Pittsburgh's starting rotation to go on the disabled list this season, joining Wandy Rodriguez, Jeanmar Gomez and James McDonald. A fifth prospective starter, Jeff Karstens, has yet to throw a pitch this season and underwent surgery on his right shoulder last week.

This is Burnett's second trip to the DL since joining the Pirates in February 2012. He fractured the right orbital bone during bunting practice shortly after joining the team and missed three weeks of the regular season but still finished 16-10 with a 3.51 ERA.

"Hopefully I can get back sooner than later and not have this turn into a six-week thing," he said. "I can take my time and keep it a two-week thing."