Walker was sent down to Triple-A Reno on Monday.
Walker will return to the minors after going 2-for-5 during his first big-league start Sunday. The 27-year-old will likely be back with Arizona later this summer, but was cast off the 25-man roster in order to make room for A.J. Pollock (thumb), who was activated from the 10-day DL in a corresponding move.