Manager Bruce Bochy said the injury has bothered Sandoval to varying degrees for several years, but it flared up Sunday when the 33-year-old switch-hitter was batting right-handed.
Bochy says Sandoval recently underwent an MRI, but the severity of his injury isn't known. He had a cortisone shot on Sunday.
"He is in pain," Bochy said. "This has flared up before, but not quite like this. Sometimes it goes away in two or three days, and other times it's lingered. This time we can't get it to go away."
A member of the Giants' three World Series championship teams from 2010 to 2014, Sandoval has been a revelation since returning to San Francisco in 2017 after being released by the Boston Red Sox. He is batting .269 with 14 homers and 41 RBIs.
Sandoval has appeared in 107 games, playing 45 times at third base, 23 at first and 49 as a pinch hitter. He has pitched once.
Sandoval is in the last year of a five-year, $95 million contract he signed with Boston in 2014, the bulk of which is being paid by the Red Sox.
In a corresponding move Wednesday, the Giants activated outfielder Alex Dickerson (right oblique strain).