"His back flared up on him," said manager Bruce Bochy, who replaced Sandoval with Nick Noonan.
Sandoval's playing status for Sunday's series finale is uncertain. He's batting .274 with 13 home runs and 75 RBIs, second on the club to Hunter Pence's 93. The switch-hitter has appeared in 135 of San Francisco's 155 games, his highest total since he played 152 games in 2010.
Chris Haft is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.