Diaz was recalled from Triple-A Indianapolis on Monday.
With the Pirates temporarily in need of another catcher while Francisco Cervelli nurses a foot injury, Diaz got the call, with John Bormann being optioned in a corresponding move. The 26-year-old is slashing .305/.317/.441 through 59 at-bats with Indianapolis this season. Cervelli remains confident that he won't need a DL stint, so if that's the case, Diaz's stay in the majors could be a brief one.