Diaz was called up from Double-A New Hampshire Saturday.
Nick Kingham (oblique) was placed on the 10-day injured list in a corresponding move. Diaz owns an 8-7 record, 4.15 ERA and 1.30 WHIP across 19 starts with New Hampshire. The 22-year-old right-hander will look to provide bullpen depth upon making his major-league debut.