Neymar was in tears as he left the field with a knee injury at the end of the first half of Brazil's 2-0 World Cup qualifying loss at Uruguay on Tuesday, and was later seen leaving the stadium on crutches.
The 31-year-old striker tripped and fell during a run in the 44th minute at the Centenario Stadium and was immediately surrounded by players from both teams as he held his left knee.
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Neymar was carried off on a stretcher as he held his hands to his face, while teammate Richarlison came on to replace him.
Brazil's medical team had no immediate update on Neymar's condition at half-time, but team doctor Rodrigo Lasmar said after the game that the star player had already undergone tests and that more would be done on Wednesday.
"These 24 hours will be important to see how the knee will respond, how will be the swelling," Lasmar said. "Imaging tests that will define a definitive diagnosis. It's too early to say [if it's a ligament injury]. Let's wait for the exams calmly, evaluate calmly, and as soon as we have a definition we will let you know."
Neymar did not speak to reporters in Montevideo. After leaving the stadium, he posted on Instagram that "God knows about all things."
"All the honor and all the glory will always be yours, my Lord. No matter what, I have faith," he said in the post.
Darwin Nunez scored the opener for Uruguay and Nicolás de la Cruz scored the second for Marcelo Bielsa's side in a match that saw Brazil drop points for the second match in a row. It suffered its first loss in 37 matches in World Cup qualifying.
Neymar, who plays for Al Hilal of the Saudi Pro League, was criticized by fans before the World Cup qualifier because of his subpar performance during Thursday's 1-1 draw against Venezuela.
"Let's hope it's nothing serious," Brazil captain Casemiro told Globo television. "He's an important player for us, we're very fond of him. He's been suffering a lot from injuries and when he starts to pick up the pace he gets injured again."
Neymar made his national team debut in 2010 and has 79 goals in 128 appearances for Brazil, but he has struggled with injuries for both club and country in recent years. He has missed multiple months in each of the past two seasons with different ankle injuries and sat out two World Cup games in Qatar after going down in Brazil's opener against Serbia.
Information from Reuters and The Associated Press was used in this story.