Giles suffered the injury in a collision with center fielder Andruw Jones Saturday in Milwaukee. In the first inning, the pair tried to run down a shallow fly ball.
Giles, who turned 26 Tuesday, will be sidelined six to eight weeks. He is 12th in the National League in hitting at .339 with three home runs and 18 RBI.
To fill the roster, Atlanta activated infielder Jesse Garcia from the bereavement list. he had been out since Friday following the death of his grandmother.
In 29 games, Garcia is batting .295 with one homer and eight RBI.
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