Jayson Tatum, especially befitting a rookie, has occasionally been prone to slow starts. He finishes much better. For a Celtics team that could use a more consistent presence next to Kyrie Irving in crunch time moments, Tatum seems to be that guy despite the greenness of his 19 years. Irving may know more about finishing off games in the post-Kobe era than anyone in the league. He also knows a future triggerman when he sees one. The point guard, thanks only in part to their common Duke roots, has been following Tatum for quite some time. Irving often talks of young players he has watched in their pre-NBA years, but Tatum has come in with a scoring skill set that can even take Irving’s breath away. “He’s a maturing individual, and he’s mature beyond his years,” said Irving. “Especially out on that basketball court, he has the ability to get to open spots and make timely jump shots as well as make drives to the basket that are kind of uncanny for a young player.