The unnamed player, an Uzbekistan international who plays for Buriram United, tested positive despite showing no symptoms, the Department of Disease Control announced on Friday.
However his team-mates all tested negative.
The 29-year-old player arrived in Thailand on August 13 and underwent two weeks of quarantine, said Dr Sopon Iamsirithaworn, director of Bureau of General Communicable Diseases.
He then tested negative for Covid-19 twice before returning to his club with a single driver on August 27.
He trained with the team until Tuesday this week, staying alone at his residence in Buri Ram. His daily schedule was usually to wake up late, go to lunch with three foreign players and then train with the team from 5pm to 6.30pm. He travelled in a teammate’s car and did not use public transport.
On Tuesday (September 8), the Department of Medical Sciences took samples from 42 footballers as part of a programme to test 1,115 Football Association of Thailand players and staff ahead of the new season starting on Saturday.
On Thursday, the team flew to Samut Prakan via Air Asia for a camping trip. The player wore a mask and stayed in his own room. The result of his test came back positive on Friday and he was taken to hospital.
Dr Yong Poovorawan, a virologist at Chulalongkorn University, said it was likely the player imported the infection from Uzbekistan. “We were quite lucky to catch this [rare symptomless] infection, which occurs in less than one per cent of cases,” he said.
He also confirmed that the 14-day quarantine was enough to detect the virus.
The start of the football season has been postponed for further testing of players and staff, said Korrawee Prissananantakul, general secretary of the Football Association Thailand.
Dr Suwannachai added that people who think they might have been in contact with the infected player should get themselves tested.