Carter suffered the injury during the River Cats' home game against Fresno on Friday while diving for a ball in the outfield, A's assistant general manager David Forst confirmed. He also sprained his right ankle in the same game, but A's manager Bob Geren noted his ankle is more of a "day-to-day thing."
X-rays were negative, but Forst said there is no timetable for the return of Carter, Oakland's highly touted prospect who received his first taste of the big leagues this month but struggled, going 0-for-19 with nine strikeouts before being optioned to work on his swing.
Carter, who reached the 30-homer mark with Sacramento this week, appeared to be an easy choice for a September roster addition with the A's. He's hitting .258 with 31 home runs and 94 RBIs for the River Cats, but his injury will likely force his Oakland return to be delayed for a handful of weeks.