The Athletics optioned Muncy to Triple-A Las Vegas on Wednesday.
Muncy is slashing .227/.303/.396 with a 33.1 percent strikeout rate this season, and his playing time has been a bit sporadic since he returned from the injured list in early June. With Muncy demoted to the minors and Tyler Soderstrom returning from the injured list, Zack Gelof should settle in as the club's primary third baseman.