The right-handed hurler allowed at least one hit in all six appearances for Cincinnati, posting an inflated 2.55 WHIP with a 9.45 ERA in 6 2/3 innings of work.
The Reed callup and Diaz demotion marks the latest in a turbulent month for the Reds' staff.
It's the fifth time this month the club has optioned or outrighted a pitcher. The Reds have sent three pitchers on rehab assignments in June and also placed A.J. Morris on the 15-day disabled list on Thursday.
Diaz has now been recalled to the big league club and subsequently sent down to Louisville twice this month.
Chris Abshire is a contributor to MLB.com based in Houston. This story was not subject to the approval of Major League Baseball or its clubs.