Pirates pitcher A.J. Burnett is wearing a walking boot over his torn right calf and isn't sure when he'll be ready to return.
Burnett injured the calf while working out Wednesday and was placed on the 15-day disabled list a day later by Pittsburgh.
Rookie Brandon Cumpton is to make his major league debut when he starts in place of Burnett on Saturday against Clayton Kershaw and the Los Angeles Dodgers.
The 36-year-old Burnett is 4-6 this season with a 4.00 ERA. He threw a season-high 112 pitches in a win over the Chicago Cubs last Saturday but said the injury is not related.
The right-hander 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.