Diaz was originally signed as a 16-year-old out of Venezuela by the Astros. The Reds, who added Diaz as a free agent before the 2016 season, are his fourth organization. Diaz has always had strong strikeout numbers, but has struggled with command at times. With the Reds, Diaz seems to have found better control, but the strikeout numbers are down. Regardless, he saw early success with Louisville in 2016. Diaz really hit his stride after joining the Red Sox organization in 2014, seeing a drastic drop in ERA, as he gradually moved from High-A to Triple-A. For having a smaller frame, Diaz has the ability to consistently get the fastball in the mid-90s. He also has a solid slider in the low- to mid-80s.