Casilla made his professional debut with the rookie league Dominican Summer League Athletics in 2000. Appearing in 11 games, 10 of which he started, he had a 6–2 record and a 3.26 earned run average (ERA). (Note: May need to click "View More Bio Info+" to see some of the information.) In 2001, he pitched in 12 games (7 starts) for the rookie league Arizona League Athletics, posting a 4–2 record, a 2.85 ERA, 50 strikeouts, 7 walks, and 37 hits allowed in innings pitched.
