Neymar left Brazil training on Tuesday after just 15 minutes with a right ankle injury that the team doctor said is related to a foul he suffered against Switzerland and not his recently broken right foot.
Neymar was playing keep-away with a few other players when he suddenly left the training session in Sochi. He limped as he walked away from the field toward the dressing room.
Doctor Rodrigo Lasmar said Neymar left as a precaution after feeling soreness while jogging. The superstar was set to undergo physiotherapy later in the day with an eye toward being fit for Wednesday afternoon's training. Brazil face Costa Rica on Friday in their second Group E game as they look to claim their first win of the tournament.
"What we can do is to calm down," Lasmar said. "A shorter workout was planned for the starters today, they're still in a regenerative process. Neymar trained a little, felt the pain, returned to physical therapy and tomorrow trains normally."
A day earlier, the forward had skipped training, one day after Sunday's 1-1 draw against Switzerland -- a game in which he was fouled 10 times, the most of any player in the competition in 20 years.
Most of the players who started against Switzerland only did light jogging on Monday, so Neymar's departure on Tuesday is of greater concern.
On Instagram on Monday, Neymar posted a picture of his foot while receiving treatment alongside the message "work hard."
Neymar, whose first season with Paris Saint-Germain ended in February after undergoing surgery to address a fractured fifth metatarsal in his right foot, made his return in the warm-up friendlies prior to the start of the tournament.
Phillipe Coutinho, who opened the scoring against Switzerland, said the players did not consider the Costa Rica clash to be a must-win match.
"We haven't spoken about [it being a must-win game]," Coutinho told reporters after training.
"We said after the last game we need to improve. All games are like a final, even more so in the World Cup. We need to take responsibility and our heads have to be in the right place. This game is important and we're looking for three points.
"We spoke about the possibilities and the ways to qualify and points. We had a meeting about that but we take to the field aiming to win."
The five-times world champions are among the favourites to lift the trophy but Coutinho warned their supporters that just because Brazil have an unrivalled pedigree, they are not going to canter to the title in Moscow on July 15.
"All games are difficult," he said. "Lots of fans think that because we are Brazil we are going to win easily and score lots of goals.
"Today everyone is better prepared for a World Cup. That was the message we took from the first game. We need to be 110 percent to win our matches."