Neymar was stretchered off in Paris Saint-Germain's 3-0 victory over rivals Marseille after injuring his right ankle in the 77th minute.
The Brazil international had been a doubt to play in the Le Classique clash on Sunday because of an illness but was named in the starting XI and contributed to two goals in the match.
But he stayed on the ground in visible pain in the final quarter-hour after a false step while tracking a Marseille player caused his right ankle to roll over despite suffering no contact.
He was taken off by medical staff minutes later and did not return.
The injury comes just nine days before PSG will try to overturn a 3-1 deficit against Real Madrid in a critical Champions League round-of-16 second-leg clash at the Parc des Princes.
However, PSG manager Unai Emery played down the severity of the injury after the match.
He told his postgame news conference: "If I had to pick my Real Madrid side now, I would say that Neymar could feature. The first look says it is a sprain. There will be further tests to be totally sure. We are optimistic."
PSG captain Thiago Silva lamented his Brazil teammate's loss, telling Canal+ that Neymar's ankle had swollen soon after the injury.
"We lost a very important player before the end in Neymar, and his ankle had already started to swell," Silva said immediately after the match.
Striker Edinson Cavani added: "Neymar? We saw what happened. It is an injury. We do not know anything more at this time, but we are all with him. We know nothing else right now."
Goalkeeper Alphonse Areola was hoping for a speedy recovery, but said that the team is prepared to cope with any injuries, even to star players.
"He is an important player and we all hope that he will be back quickly," Areola said. "However, I am not a doctor and I do not know how long it will take him to return and if it will be soon.
"Every player is important and we know that we have the necessary quality to deal with inevitable injuries."
PSG will next face Marseille in the Coupe de France quarterfinals on Wednesday.