Mountcastle went 0-for-3 with a walk and a stolen base in Wednesday's 3-0 loss to the Marlins.
Mountcastle drew the start in left field and hit third Wednesday. In the fourth inning, he drew a walk and stole second, but he was left stranded. The 24-year-old is slashing only .169/.203/.277 with a homer, seven RBI, six runs scored and two stolen bases across 69 plate appearances. He's in a 0-for-18 rut at the plate. The Orioles begin a three-game series with the Athletics on Friday, and the return to AL play enables the utilization of Mountcastle as the designated hitter.