Tatum announced the arrival of his son on Friday on social media.
"Jayson Christopher Tatum Jr.! God's greatest gift to me! Daddy loves you more than anything in the world man! Truly a blessing," he wrote on Instagram.
Tatum, 19, shared photos of his son, wearing Duke booties and a hat in a nod to his former school.
The Celtics rookie did not reveal the exact birthdate of the baby or who the mother is.
Tatum was the No. 3 overall pick in the 2017 NBA Draft. He is averaging 13.9 points and 5.5 rebounds per game this season for Boston. Tatum tweaked his knee during Thursday's 114-103 victory against the Philadelphia 76ers at the O2 Arena in London. ESPN reported that the injury occurred in the third quarter of the matchup.
The NBA's Eastern Conference Rookie of the Month for December has been held out of practice as he has the left knee evaluated.
"I didn't see it," Celtics coach Brad Stevens told reporters Saturday, according to the Boston Herald. "but they said that he got his knee hit and it stiffened up, swelled up a little bit the next morning. But he's going to have further testing done."
"They don't think it's a big, big thing, but certainly we want to be cautious and want to know exactly what happened."
"We're going to get more imaging on [it]. We don't know what's going on, but it was stiff today, so we wanted to make sure we held him out."
The Celtics host the New Orleans Pelicans at 7:30 p.m. Tuesday at TD Garden in Boston.