Paul Pierce welcomes his newborn son, Prince Paul Pierce

Paul Pierce had a very good reason, it turns out, to miss Monday night’s game.

While the Celtics were in Minnesota, Pierce was home in Boston with his wife, who was about to give birth.

And at 8:21 p.m. Tuesday, the couple’s first son, Prince Paul Pierce, entered this world.

“In the next 18 years, world, Celtics fans, look out,” Pierce told WEEI late Wednesday night after the Celtics beat the Detroit Pistons, 98-93, at TD Garden.

With their win and with Philadelphia’s loss at Charlotte, the Celtics clinched a playoff berth for the sixth consecutive season.

Pierce scored 17 points and had five assists in 33 minutes Wednesday.

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