Underwood was removed from Thursday's game against the Cardinals due to right hamstring discomfort, Justice delos Santos of MLB.com reports.
Underwood entered Thursday's Opening Day matchup in the bottom of the sixth inning, but he was removed after allowing a run while failing to record an out with 10 pitches. It's not yet clear whether the right-hander's injury will force him to miss additional time, but he'll be able to rest during Friday's scheduled off day.