De Leon was recalled from the Reds' alternate training site Thursday.
De Leon wasn't part of the Reds' 40-man roster while working with the team during summer camp, but he'll join the major-league bullpen after Brooks Raley was designated for assignment in a corresponding move. The right-hander returned to the mound in 2019 after undergoing Tommy John surgery in 2018. However, he pitched just four innings in the majors with the Rays, posting a 2.25 ERA and 7:3 K:BB during that time. Given his small sample in the big leagues in past seasons, De Leon should occupy a lower-leverage role in the bullpen.