Pivetta (forearm) was removed from his rehab start with Single-A Lake Elsinore on Tuesday due to an apparent injury, AJ Cassavell of MLB.com reports.
The right-hander was making his first rehab appearance since going down with a forearm strain in mid-April, but he exited Tuesday's game during the middle of an at-bat after throwing a pitch in the fourth inning. The specifics of the situation are unclear, but any sort of significant setback could threaten Pivetta's availability for the rest of the season. The Padres will likely provide an update in the near future.