Pirates starting pitcher A.J. Burnett has been placed on the 15-day disabled list with a right calf strain, the club announced Thursday afternoon.

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 just yet. 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.

Starting pitchers Wandy Rodriguez, Jeff Karstens and James McDonald are already on the DL for Pittsburgh.

In order 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.