Diamondbacks manager Kirk Gibson said Monday that Harrell will join Triple-A Reno for now.
The 28-year-old Harrell went 0-3 with a 9.49 ERA in three starts with Houston this season. He was 6-17 with a 5.86 ERA last season.
Arizona could use some pitching help.
The Diamondbacks lost ace Patrick Corbin to Tommy John surgery before the season started and are off to the worst start in franchise history, though the starters have been solid the past four games.