United States striker Jozy Altidore has been hospitalised with a sprained ankle.
Villarreal striker Altidore missed practice on Thursday due to the injury but American broadcaster ESPN said hospital X-rays ruled out any serious damage. The reported added that Altidore's fitness was being assessed on a day-to-day basis.
The 20-year-old, who spent last season on loan at Hull, had been expected to lead the US attack against England in their opening World Cup game in Rustenburg on June 12.
The US Soccer Federation confirmed Altidore's injury with a short statement posted on its website, citing the injury as a mild ankle sprain.
"Jozy Altidore sat out training after incurring a mild right ankle sprain during Wednesday's practice," the statement on ussoccer.com read.
"Altidore's status is considered day-to-day, and he will undergo a fitness test Friday to determine his availability for the June 5 friendly against Australia."