The incident came in the sixth inning of the Giants' 2-1 loss to the Arizona Diamondbacks. Belt stayed down for several moments while being attended to by trainer Dave Groeschner. Diamondbacks pitcher Anthony Banda knelt on the mound with his face in his glove.
Belt was examined again Saturday and placed on the seven-day concussion disabled list.
"It's a curveball that caught him flush," Giants manager Bruce Bochy said. "He's been through this before, so it's obvious there are some concerns there. Our hope is this thing's going to be a lot better tomorrow."
It is Belt's fourth concussion in eight years.
He is currently batting .241 with a team-leading 18 home runs and 51 RBIs.
The Associated Press contributed to this report.