Mountcastle is out of the lineup for Friday's game against the Brewers, Roch Kubatko of MASNSports.com reports.
Mountcastle started the first 12 games of the season and has an .832 OPS with five doubles in 55 plate appearances, and he'll receive a well-deserved rest day. Ryan O'Hearn will take over at first base while Anthony Santander serves as the designated hitter.