Jayson Tatum hurt his left knee in the third quarter vs the 76ers on Thursday.
Brad Stevens said Tatum's knee stiffened and swelled up the following morning, and the 19-year-old rookie would undergo further testing.
Tatum sat out of the Celtics' practice session, but no other details about the injury have been released. Stevens told reporters, "They don't think it's a big, big thing, but certainly we want to be cautious and want to know exactly what happened."