The Diamondbacks optioned Diaz to Triple-A Reno on Thursday.
Diaz made three appearances for the Diamondbacks after being recalled from Triple-A on Sept. 12, during which he allowed three earned runs over 8.1 innings while striking out five batters and walking four. He and Blake Walston will now be sent back to the minors as Alek Thomas comes up from Reno and Ryne Nelson (shoulder) returns from the IL.