The striker asked to leave practice after just 20 minutes with a lot of pain in his right knee, and went out immediately to get treatement on the issue.
Reports from the scene, however, say that it was just a scare, and Neymar himself told his closest teammates that there is no serious problem, and he's not at risk of having an actual injury. But because of how close the next match is (on Saturday afternoon against Costa Rica), there is a high probability that coach Dunga doesn't play Neymar as a precautionary measure.
Neymar didn't have a preseason after being given longer vacation due to his participation in the Copa América, and he then had to deal with a parotitis that sidelined him for two weeks at Barcelona. He then played 90 minutes against Málaga last week, and the strong rythm of training might have an effect on this problem. Luckily, it's nothing serious.