Harrell, 28, is 0-3 with a 9.49 ERA and 2.27 WHIP through three starts this season. He was 6-17 with a 5.86 ERA last season after putting together a decent 2012 season (3.76 ERA in 193 2/3 innings). The 6-foot-2 right-hander was a fourth-round pick out of high school by the White Sox in 2004.
At Harrell's age and considering his dismal performance in the past two seasons, he obviously didn't fit into the Astros' future plans.
As far as the Diamondbacks, they've gotten awful starting pitching for the most part this season with Patrick Corbin out. It seems doubtful Harrell would be immediately thrown into the rotation, though, and instead he'll be a depth piece with the hopes that a change of scenery can help him get back to 2012 form.