Denis Cheryshev, who scored four goals for Russia at the World Cup, is under investigation for an alleged doping violation.
The 27-year-old, on loan at Valencia from Villarreal, fell under suspicion after his father revealed his son had used growth hormone (HGH) treatment.
The World Anti-Doping Agency told BBC Sport the case was being handled by the Spanish anti-doping authorities.
Cheryshev played five World Cup games as hosts Russia reached the last eight.
The taking of HGH without a therapeutic use exemption can lead to a four-year ban.
Cheryshev’s father later said that his remarks, made before last summer’s World Cup, had been misquoted by a journalist.
Russian sports daily Sport-Express quoted the player, who began his career at Real Madrid, as saying: “On my side, everything has been honest and I think there will be no problem.
“I think we’d better leave this case in the hands of medics, who have done everything right.”