Giants manager Bruce Bochy said Thursday that Sandoval has a significant tear requiring surgery. Sandoval was injured while sliding into home plate Sunday against San Diego.
Sandoval, 31, was hitting .248 with nine homers and 40 RBIs. He also played first and third base during his second stint with San Francisco.
Sandoval played his first seven seasons with the Giants, winning World Series MVP honors in 2012. He became a free agent and signed a $95 million, five-year contract with Boston after the 2014 season.
He was released by the Red Sox in 2017 and signed with the Giants three days later. San Francisco is paying the $545,000 minimum this year, which is offset against the $18 million Sandoval is owed by Boston.