USA coach Gregg Berhalter praised Chelsea attacker Christian Pulisic after victory over Iran.
Pulisic proved the matchwinner and Berhalter confirmed after the game he had been taken to a local hospital in Qatar as a precaution after experiencing some dizziness.
The US head coach wouldn't speculate on his availability for Saturday's knockout game, but he did highlight Pulisic's commitment to the American cause.
"I've been saying all along it's a wonderful thing when one of your best players is also one of the hardest working," said the head coach. "He's certainly that.
"I can't say enough positive things about Christian."