Sisk (elbow) will begin a rehab assignment at Triple-A Indianapolis on Thursday, Alex Stumpf of MLB.com reports.
Sisk has been shelved for the past month with left elbow inflammation, but he's progressed enough to pitch in games again. The southpaw reliever was terrific out of the Pirates bullpen before getting hurt, collecting a 2.23 ERA, 1.18 WHIP and 43:13 K:BB in 36.1 innings.