Underwood will start Saturday's Cactus League game against the Angels with Alec Mills scratched due to a back issue, Mark Gonzales of the Chicago Tribune reports.
Underwood has thrown just a single inning in Cactus League action so far, but the 24-year-old will get a chance for some extended work Saturday. He made one start for the Cubs last season and will likely begin 2019 at Triple-A with a chance to move up if there are any injuries in the Cubs rotation.