Diaz (biceps) will throw a bullpen session Sunday, Bobby Nightengale of The Cincinnati Enquirer reports.
Diaz is scheduled to throw live BP on Wednesday following Sunday's throwing session, so he should remain on the 15-day injured list until at least late next week. The right-hander could take on a setup role once he returns to action since Hunter Strickland has performed well as the Reds' closer recently.