Underwood (hamstring) allowed one hit and one walk while striking out two across a scoreless inning during a rehab appearance with Double-A Altoona on Tuesday.
Underwood also threw two scoreless innings with Triple-A Indianapolis on May 7, but his rehab assignment was transferred to Altoona given the geographic proximity to Pittsburgh. In total, he's thrown 4.2 innings while working back from the hamstring issue and will likely be activated from the injured list in the coming days. Underwood figures to occupy a high-leverage role in the Pirates' bullpen, but he won't be in the mix for saves without a number of injuries ahead of him.