Burnett, 36, has an ERA of 3.12 over 89 1/3 innings this season for the Pirates. He has made at least 30 starts every season since 2008.
The team also reinstated starting pitcher Charlie Morton from the 60-day disabled list, and unconditionally released Jose Contreras. Morton could be a candidate to make the start on Saturday that has been vacated by the flurry of roster moves.
Contreras had an ERA of 9.00 in five innings so far this season, so it comes as no surprise to see him be the odd man out. At age 41, he may have thrown his last pitch in the major leagues.
The Pirates, who have surprised everyone early on with their excellent play, will face likely their toughest test in the form of these injuries. Burnett has been a solid veteran presence in their rotation, and they will miss the steadiness that he provided. It sounds like his injury is not too bad, but in the meantime someone will have to step up in his absence.