Underwood (hamstring) is scheduled to join Triple-A Indianapolis over the weekend to continue his rehab assignment, Mike Persak of the Pittsburgh Post-Gazette reports.
Underwood has made two appearances with Single-A Bradenton, walking two with no strikeouts across 1.2 innings. He's been sidelined since Opening Day, so he's understandably showing some rust. He'll likely need a few more rehab appearances but is scheduled to be evaluated for a potential return Monday.