Sergio Aguero reportedly fainted during half-time and was taken to hospital as Nigeria stunned Argentina in a friendly.
The Manchester City striker fell ill in the changing room. He was replaced by Dario Benedetto.
Aguero had earlier scored to put his side 2-0 up, becoming the Albiceleste's third-highest scorer in the process.
But he suffered health issues at the break and was forced to leave the Stadion FK in Krasnodar to be given medical attention.
He later recovered consciousness, according to TyC Sports.
Aguero underwent tests in hospital as Nigeria came back to record a famous 4-2 win.
Arsenal's Alex Iwobi struck twice for Nigeria, including a stunner as he nutmegged Barcelona's Javier Mascherano before slamming home.
Aguero has eight goals in eight Premier League appearances for City so far this season, and his condition will be of huge concern for boss Pep Guardiola.
The Blues go to Leicester on Saturday.