Diaz was optioned to Triple-A Indianapolis on Thursday.
Diaz finds himself behind Francisco Cervelli and Chris Stewart on the depth chart heading into the 2017 season, so the Pirates will send him to the minors to see more consistent playing time. The 26-year-old slashed .266/.289/.298 in an injury-plagued 2016, where he was limited to just 25 games at the minor league level.