The Diamondbacks reinstated Walston (elbow) from the 60-day injured list Tuesday.
Walston has been on the injured list all season while recovering from a Tommy John procedure he underwent last spring. He was cleared to begin a rehab assignment Aug. 4 and has since given up four earned runs in 7.2 innings at Triple-A Reno. The 25-year-old lefty will join the D-backs' bullpen as a long reliever now that he's fully healthy, though he could still be developed as a starter in the future. Ryan Thompson (back) was placed on the 60-day IL in a corresponding move.