Per NBC Sports Bay Area's Alex Pavlovic, Posey was removed in the third inning after recording a single. He took a foul ball off his catcher's mask during the first inning.
The Athletic's Andrew Baggarly noted it was possible Posey experienced symptoms after running down the first-base line prior to his exit.
The 31-year-old became an All-Star for the sixth time in 2018, though he didn't represent the National League in the All-Star Game itself. He sat out the event in order to deal with a lingering hip injury.
The injury could help explain Posey's somewhat underwhelming offensive numbers compared to his production in recent years. Through 90 games, he's batting .293 with five home runs and 37 RBI.
By comparison, Posey averaged 15 homers and 81 RBI with a .308 batting average over his previous three seasons, per Baseball Reference.
The Giants are hanging on the edge of playoff contention with a 56-54 record. They trail the Los Angeles Dodgers by five games in the National League West.
Nick Hundley will take over for Posey as San Francisco's primary catcher. The 34-year-old is slugging .475 in 56 games this season.