As to how Burnett suffered the injury, the Pirates aren't saying. Manager Clint Hurdle did tell reporters, however, that Burnett informed them of the issue Wednesday night.
It is unknown how the Pirates will deal with Burnett's absence. Charlie Morton has been activated and is making his first start of the season Thursday night, Jeff Locke will move up to start Friday and Gerrit Cole is slated to go Sunday. There's no chance the Pirates make prized prospect Cole go on short rest, so a starter is needed for Saturday.
To make room for Morton on the 40-man roster, reliever Jose Contreras was given his unconditional release.
Burnett, 36, is 4-6 with a 3.12 ERA, 1.13 WHIP and an NL-best 99 strikeouts in 89 1/3 innings.