Diehl was reassigned to minor-league camp Sunday, Mandy Bell of MLB.com reports.
Diehl signed as a non-roster invitee in February and didn't allow a run in four appearances during spring training, but he wasn't able to make Cleveland's Opening Day roster. The left-hander has a 9.47 ERA in 21 career big-league outings, and he's unlikely to be more than a low-leverage option should he see the majors this season.