Manchester City forward Sergio Aguero was rushed to a local hospital in Krasnodar after he fainted in Argentina's dressing room during half-time of Tuesday's friendly with Nigeria, the Argentine Football Association (AFA) confirmed.
"Aguero suffered a lypothymia (fainting without loss of consciousness), which is why he was transferred to hospital for routine tests as a precaution," the AFA said in a statement.
Aguero, who scored the winner in Argentina's 1-0 victory over Russia on Saturday, netted his team's second goal against Nigeria in the 36th minute before he was replaced at the break after suddenly feeling ill.
He was taken by team doctor Donato Villani to a clinic in Krasnodar.
"He felt bad twice, and with the medical history he has, since he was operated on when he was 15, we wanted to be sure it was nothing," said Villani, referencing a previous heart problem.
"We took him to hospital to make sure everything was normal, we prefer to wait to be sure."
Aguero, Manchester City's record goalscorer, had a health scare less than two months ago when he suffered a broken rib in a car accident in Amsterdam.
Nigeria battled back from two goals down to beat Argentina 4-2 in Tuesday's game.