Posey, who was previously reported to be dealing with a thumb issue, has gone 2-for-24 (.087) in his past six games, with a .125 on-base percentage. Posey was 0-for-4 in the series opener at Anaheim on Friday.
Since April 9, Sandoval has started just one game, going 0-for-3 at Arizona on Tuesday. The veteran is just 4-for-21 (.190) this season with a .261 on-base percentage.
Manager Bruce Bochy had targeted Sandoval for a probable start in Sunday's series finale against the Angels and possibly at the DH spot.
The severity of Posey's injury was not immediately known. Sandoval remained in the cleanup spot that Posey had previously occupied.