Assistant coach Dan Craig, who grew up about 40 minutes away from TD Garden in Chelmsford, directed the Heat.
"In the morning, the coach got the message from his wife that she was having contractions," Craig said. "He got on flight early this morning, fortunately got back there safely and he's with her now."
Spoelstra's wife, Nikki, posted a photo to Twitter on Thursday of the couple along with their newborn, Dante Nicholas Spoelstra.
Welcome to the world, Dante Nicholas Spoelstra! Perfect in every way. So grateful to have this big, magical soul join our family & can't wait to bring him Home to the best big brother & doggie-sister ever! You are loved by so many already, Baby Dante!— Nikki Spoelstra (@NikkiSpo) December 5, 2019
Craig, 37, rooted for the Celtics when he lived in the area. He grew up in upstate New York, but his family moved north of Boston when he was 12.
"It worked out kind of funny," he said before Wednesday's game. "I'm back home and I'm going to coach this game. It's pretty exciting."
In March 2018, Craig also coached the Heat when Erik Spoelstra was with Nikki for the birth of their first son. The Heat also lost that game.
The Associated Press contributed to this report.