Belt, who missed 12 games with a concussion before being activated on Aug. 2, left the game Wednesday with headaches.
Belt, who is hitting .237 with 11 home runs and 24 RBIs in 51 games, first went on the concussion list July 19 when struck in the face by a throw in batting practice in Miami.
''We'll have to see how he comes around this time, whether it's fast or slow, but we do have concerns,'' Giants manager Bruce Bochy said.
The Giants recalled infielder Matt Duffy, who was optioned Thursday to the minors and started him at second base against the Kansas City Royals.
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