Belt left in the seventh inning of Wednesday's 3-2 loss to the Mariners with a hyperextended knee.
He was hurt reaching on an infield single when he took a hard step after reaching first base. He limped off the field after being briefly examined by a trainer.
Longoria suffered a fracture in his left hand last month.
The Giants also recalled pitcher Chris Stratton from Triple-A Sacramento and recalled reliever Ty Blach, who was sent to Triple-A on Wednesday.