Argentina and Manchester City forward Sergio "Kun" Aguero fainted in the dressing room at the half of his national side's 4-2 loss to Nigeria on Tuesday in Krasnodar, Russia.
Aguero, 29, regained consciousness at a local hospital where he remained for testing.
Playing for the 84th time for Argentina, Aguero scored his 34th international goal in the first half of the friendly to make it 2-0 after Ever Banega's opener.
Aguero's former teammate at City, Leicester's Kelechi Iheanacho, pulled one back for Nigeria just before the half.
Nigeria struck three times in the second half, twice through Arsenal's Alex Iwobi and once from Brian Idowu, to come back for the victory.
Aguero has made eight Premier League appearances for league-leading City, scoring eight times.
City returns to Premier League action on Saturday when it visits Leicester at the King Power Stadium.