"He complained to [head athletic trainer] Dave Groeschner that he had trouble concentrating and couldn't focus, so we had to take him out," Giants manager Bruce Bochy said of Belt on Friday.
Belt was noticeably shaken up during a play Tuesday night against Cincinnati when he dove into second base and hit his head against an infielder's knee. Bochy said Belt subsequently underwent concussion tests and passed them.
The 27-year-old underwent further testing Saturday morning, when it was revealed he did indeed have a concussion.
"Belt, right now, he's going to take a little time off," Bochy said. "He's got a concussion. He showed symptoms [Friday] and that's what we're treating him for. He feels about the same, I think. He wasn't going to play [Saturday] regardless."
Belt played Wednesday and finished 1-for-4. Following Thursday's scheduled off-day, Belt went 0-for-3 Friday, which ended an eight-game hitting streak, before he departed. Nick Noonan replaced Belt at first base and struck out in his only at-bat.
The Giants will play it safe with Belt, who sustained a concussion last year when Marco Scutaro accidentally hit him with a throw July 19. That mishap caused Belt to miss 46 games.
"They're so hard to figure out, how long to take," Bochy said of concussions. "It's obvious you want to be cautious, but how cautious to be? When they pass all of the tests and clear those, you can only assume they're good to play."
Belt was batting .280 with a career-high 18 home runs, 68 RBIs, a .358 on-base percentage and a .481 slugging percentage prior to the injury.
"He's going to need some time," Bochy said. "Now, he's got to be cleared to play, and until then, he won't play."