Soriano (elbow) began a rehab assignment with Low-A Bradenton on Thursday, Kevin Gorman of TribLive.com reports.
The Pirates grabbed Soriano in the Rule 5 draft despite the fact that he was still working his way back from Tommy John surgery. He's still on the 60-day injured list and may need a lengthy rehab assignment before he's ready to make his big-league debut, but the Pirates will have to keep him on the active roster once that's done or offer him back to the Angels.