McCullers (elbow) threw off the mound Monday, Chandler Rome of the Houston Chronicle reports.
The session is McCullers' first off the mound since undergoing Tommy John surgery back in November. He'll only throw fastballs, but it still represents a significant milestone. He won't have time to return this season, but the fact that he's throwing several months before next season's spring training bodes well for his readiness for next year.