McCullers (forearm) was placed on the 15-day injured list Thursday, retroactive to March 27.
Astros GM Dana Brown said earlier this week he wasn't sure if McCullers would begin the season on the 60-day IL, but the club is apparently content to put him on the 15-day IL for now. The right-hander is dealing with a low-grade forearm strain and isn't expected to be ready for game action until May, at the earliest.