Sanchez hurt his knee in Tuesday night's 9-0 win over the Atlanta Braves. An MRI on Wednesday confirmed the sprain.
The Giants recalled catcher Eli Whiteside from Triple-A Fresno to move into Sanchez's role as Buster Posey's backup. Posey started at first base Tuesday night before moving back to catcher after Sanchez left the game in the fourth inning.
Posey is starting at catcher against the Braves on Wednesday night.
Sanchez was hitting .281 with two homers and 24 RBIs. Whiteside has a .218 career batting average in four seasons with Baltimore and San Francisco. He hit .197 in 82 games with the Giants last season.