Croatia defender Domagoj Vida was informed that he tested positive for Covid-19 after playing the first half of a friendly with Turkey in Istanbul on Wednesday, the Croatian football federation (HNS) said.
All the Croatia squad and staff members tested negative on Monday but additional testing was carried out on Wednesday morning ahead of Saturday's Nations League match with Sweden, HNS said.
The Croatia team's medical service received initial information on "one potentially positive result" during half-time of the Turkey match.
Doctors isolated the player while the suspicious result was retested and confirmation came that he was positive.
Vida, the 31-year-old World Cup finalist who plays club football for Turkish side Besiktas, will "spend the next few days in self-isolation in Istanbul", HNS said.
He is set to miss the Sweden game and the Nations League fixture against Portugal on Tuesday.
Apart from Vida one member of HNS delegation, who was not in contact with players or staff members, tested positive.
Wednesday's friendly with the Turks ended in a 3-3 draw. Croatia, who were surprise finalists at the 2018 World Cup, are third in Nations League Group A3 which also includes France, the team that beat them to the World Cup, Cristiano Ronaldo's Portugal and Sweden.