Sandoval believes he hurt the wrist Sunday, manager Bruce Bochy told reporters. He came to work Tuesday after a day off and reported soreness in the hand.
An X-ray Tuesday didn't reveal the break, but Sandoval then underwent an MRI exam and CT scan that revealed the fracture. He's scheduled to have surgery Friday at Stanford.
"Tough news for us and Pablo," Bochy told reporters. "I feel for him, particularly the way he has been playing and swinging the bat. You just don't replace a bat like Pablo's. There are few guys in Major League Baseball who can do the things he can do with the bat."
Sandoval is hitting .316 and his five home runs, 14 RBIs and 16 runs scored are all team highs.
Sandoval thanked fans for the support via his Twitter account.
Conor Gillaspie was called up from Triple-A Fresno to replace Sandoval.