The Pirates optioned Castro to their alternate training site Tuesday.
The 21-year-old infielder appeared in one game during his first cup of coffee in the majors, going 0-for-3 in an April 21 loss to the Tigers. He'll likely report to Triple-A Indianapolis or Double-A Altoona when the minor-league season begins next week, and he'll likely remain on the farm for much of the 2021 campaign unless the Pirates' infield depth is tested.