Paul Pierce had a very good reason, it turns out, to miss Monday’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 Pistons, 98-93, at TD Garden. With their win and 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. He did not reveal the birth in the locker room when he met with reporters, instead waiting to spill the beans on the radio. Rondo sighting In shorts, sneakers, a long-sleeved shirt, and a bulky brace on his right knee, All-Star point guard Rajon Rondo returned to TD Garden’s parquet floor Wednesday to get up shots about an hour before the Celtics played the Pistons. Rondo’s mere presence caused a stir, with onlookers starting to wonder if he might be making a miraculous return to the lineup a little more than two months after tearing his right anterior cruciate ligament.