The Hawks quickly ruled him out for the remainder of the night.
Gallinari played just three minutes with five points and one rebound before the injury forced him to leave the game.
The Italian joined the Hawks this offseason via sign-and-trade with the Oklahoma City Thunder.
OKC received a conditional 2025 second-round pick, Gallinari got a three-year, $61.4 million contract, and the Hawks gained some much-needed depth up front to pair with Trae Young and Cam Reddish in the backcourt.
The 32-year-old is likely to see his minutes divided between John Collins, De'Andre Hunter and Solomon Hill, but he'll be tough to replace overall.
Gallinari averaged 18.7 points and 5.2 rebounds for the Thunder last season while starting 62 games.
Along with the Orlando Magic, the Hawks (3-0) entered Wednesday as one of two teams without a loss this season.