Diaz was recalled from Triple-A Jacksonville on Friday.
Diaz was demoted to Triple-A in mid-June but will rejoin the Marlins coming out of the All-Star break. The 25-year-old is hitting .146/.286/.223 with a 29.4 percent strikeout rate in 126 plate appearances in the majors this season. He's unlikely to play a significant role with Jazz Chisholm, Jon Berti and Joe Panik healthy.