Christian Pulisic was substituted because of injury in second-half stoppage time of AC Milan's 2-1 win over Paris Saint-Germain in the Champions League on Tuesday.
The United States attacker was attempting to run down a loose ball when he pulled up and grabbed his right leg before going down to the ground.
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Pulisic had played a solid game for Milan, who beat PSG 2-1 at the San Siro, before being substituted, with Stefano Pioli's team picking up a big win in their quest to advance to the round of 16 from a competitive Group F.
Former Inter Milan defender Milan Skriniar had put PSG ahead, but their advantage lasted only three minutes before Rafael Leão netted an acrobatic equaliser and Olivier Giroud then sealed a precious victory just after the break in front of 75,649 home fans.
The U.S. has a pair of big matches later in November against Trinidad and Tobago in the Concacaf Nations League with a spot in next summer's Copa America on the line.
Gregg Berhalter is expected to name his team for the games against T&T on Thursday, with the Americans hosting the first match on Nov. 16, before travelling to Port of Spain for the away second leg.