Diaz was optioned to Triple-A Syracuse on Tuesday, Anthony DiComo of MLB.com reports.
Diaz made three relief appearances after he was called up in early September, but he'll head back to the minors after he gave up four runs on five hits and two walks while striking out one in 1.1 innings against the Cardinals on Monday. He's made 12 outings in Albuquerque and has posted an 8.16 ERA and 1.63 WHIP in 14.1 innings.