The source said the team will follow league protocols for Covid-19. The player was not identified.
A reporter for The Athletic and Stadium was the first to report the positive test, adding that sources said the 76ers learned of the positive test during the game.
Before the game, the team said it would not practice on Friday. The 76ers are scheduled to play the Denver Nuggets in Philadelphia on Saturday.
The 76ers lost to the Nets, 122-109. They are 7-2 so far in the season.