Walston was placed on the 15-day injured list Thursday due to left elbow inflammation, Alex Weiner of ArizonaSports.com reports.
Walston was called up to make his first MLB start May 27 and made an additional start Tuesday. He didn't factor in the decision in either start, but he gave up just two earned runs in nine innings. He seemed likely to stick in the rotation prior to the injury. The Diamondbacks selected Humberto Castellanos from Triple-A Reno in a corresponding move.