The forward was in action in the World Cup qualifying match with Uruguay on Tuesday until he tripped and fell just before half-time.
The incident came as he jostled for possession with Nicolas de la Cruz, with his knee appearing to buckle under him, leading to immediate concern.
Neymar was immediately surrounded by players from both teams as he screamed in agony and writhed around on the floor.
After a pause in play for treatment, the 31-year-old was stretchered off the pitch with his head in his hands, before using crutches as he left the stadium.
Speaking after the game, Rodrigo Lamar's doctor said: "We did all the tests and we will repeat them tomorrow.
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"Those 24 hours are key to see how his knee will respond, how swollen it will be and what images will show.
"Let's wait for the exams with calmness, evaluate with calmness and tell you once we have a diagnosis."
While the extent of his injury remains to be seen, Neymar could face a long spell on the sidelines if he has suffered ligament damage, putting his place in next summer's Copa America in doubt.
"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."
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As if Neymar's injury wasn't bad enough, Brazil ended up losing 2-0 to Uruguay in Montevideo, leaving them in third spot in the CONMEBOL standings with Colombia and Argentina up next.