Nelson (back) began a rehab assignment Friday with Triple-A Reno and retired two batters while working around a hit and two walks in his first appearance.
Nelson was placed on the 15-day injured list Aug. 2 with lower-back spasms and will likely require at least one more rehab appearance before being activated. Prior to being shelved with the back injury, Nelson posted a 1.57 ERA, 0.90 WHIP and 27:8 K:BB in 34.1 innings with Arizona while picking up one win and eight holds.