Castillo (adductor) will throw a bullpen session Tuesday, Alex Weiner of ArizonaSports.com reports.
Castillo exited his first Cactus League appearance Feb. 22 with left adductor tightness and hasn't pitched since. He's ready to ramp up his throwing again but it's not clear when he might be available to pitch in a game. Castillo is in camp as a non-roster invitee, and the injury likely eliminated any chance he had to make the Opening Day roster.