Sandoval believes he actually hurt the wrist Sunday, manager Bruce Bochy told reporters. He returned to work Tuesday after a day off and reported soreness in the hand.
"Tough news for us and Pablo," Bochy said. "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."
An X-ray on Tuesday did not reveal the break of the hamate bone, but Sandoval, who came out of Wednesday's game against the Miami Marlins in the seventh inning, then underwent an MRI exam and CT scan that showed the fracture. He will have surgery on Friday at Stanford and is expected to miss four to six weeks.
"He doesn't remember doing it on one swing," Giants athletic trainer Dave Groeschner said. "He was hitting right-handed Sunday and the off day Monday, then Tuesday afterward he said it was sore. X-rays are tough on that area. It's not a good way to view it."
In entries Thursday on his Twitter account, Sandoval thanked his fans for their support in what he said was a tough time.
The three entries, when strung together, read: "To all my fans ; I will like to thank you guys for all the good wishes , this is a hard moment in my career , but I know that with God blessing this will benefit me , to perform better for you guys . Thanks for all the support I will be back soon and the #PANDAMODE WILL ALWAYS BE ON NO MATTER WHAT. Thanks --- Support the team everyone"
Conor Gillaspie was called up from Triple-A Fresno to replace Sandoval on the Giants' roster. He was added to the starting lineup and was due to bat second and play third base Thursday afternoon against the Marlins.
Sandoval is hitting .316, and his five home runs, 14 RBIs and 16 runs scored are all team highs.
The injury comes almost exactly a year after Sandoval fractured the hamate bone in his right hand. That injury resulted in surgery, and he missed 41 games.
The Giants expect Aubrey Huff to come off the DL on Monday after his battle with anxiety disorder. Huff has been working out and taking batting practice, while giving Bochy indications he is ready both physically and mentally to get back on the field.
Information from The Associated Press was used in this report.