"Yesterday he started showing some symptoms; we had some concerns," Giants manager Bruce Bochy said, adding, "It's important to take care of Brandon. The symptoms are there again."
Belt was struck in the face with a thrown ball in batting practice July 19 and subsequently was placed on the seven-day DL. He returned Aug. 2 to play the entire Mets series, then the first two games against the Brewers. In the second game, Belt went 0-for-5 with four strikeouts. He sat out the finale.
"He was doing well there. He went four games ... playing at a high level," Bochy said.
After being optioned to Double-A Richmond on Thursday, Matt Duffy was recalled to take Belt's roster spot. Sensing the severity of Belt's headaches, the Giants had Duffy travel with the team to Kansas City.
Michael Morse started at first base for Belt on Friday.
Belt's injury comes one game after the Giants got leadoff hitter Angel Pagan back.
Pagan, absent from the lineup since June 15 with a strained back, collected a couple of hits in his return Thursday.
"Getting Angel back, I think everybody was excited to see the lineup that we envisioned," Bochy said. "We've got another little setback, but you know what, nothing you can do but move forward."
Jackson Alexander is an associate reporter for MLB.com.