Christian Pulisic came off in the 59th minute of the United States' 4-1 loss to Belgium after twisting his right ankle on a shot attempt in the 52nd.
The injury happened when Pulisic extended for the ball and struck Belgium captain Youri Tielemans' boot, then stayed down briefly and began favoring the foot.
Mauricio Pochettino said he hoped the problem was not serious, with Pulisic due to rejoin AC Milan after the standard 21-day post-World Cup break.
That exit capped a frustrating tournament for the U.S. star, who was held scoreless in this World Cup and had already been limited by a calf injury that cost him one match and part of another.