Jayson Tatum, all 19 years of him, made 3-of-5 shots from 3-point range yesterday, the last of them a clutch dagger from the right sideline to stretch the Celtics lead to eight with 1:55 left in what became a 91-81 victory over the Detroit Pistons. In so doing, Tatum cemented his league lead in 3-point percentage to .523. Pistons coach Stan Van Gundy smiled before the game when asked about the rookie. “OK, I’m not surprised,” Van Gundy said. “I thought he was the best prospect in the draft. He’s got all the tools. But what’s changed . . . anybody who says they’re not surprised by his 3-point shooting based on what he did in college is lying. I mean, if there’s somebody who said, ‘Oh, look, I knew he’d knock down 50 percent of his 3’s,’ even though he made 32 percent of them from the college line, they’re lying. And I’m sure there’s somebody saying that, that they knew. But they’re lying. “But everything else was there — his ability to put the ball on the floor and get shots. He’s got a real poise to him. He’s better defensively than people think. He’s long. He’s really, really good, and I’m sorry he’s there for the next however many years.” Cracked Brad Stevens when told of Van Gundy’s comments, “He’s probably called me a liar before. No, I mean there’s only so many people that shoot it at that level. Even though he’s shooting it great, obviously that level is reserved for great, great shooters. So you wouldn’t predict that with any young guy.” As for getting Tatum to take more 3’s, Stevens said, “I think one of our challenges this summer was everybody wanted to talk about the mid-range plays he made in summer league. So it was one of those things where, if I was involved in a film session at all this summer, it was about what a good shot looks like when you get to the NBA. And so we tried to have those very specific film sessions, but still you can’t predict a guy is going to become that proficient.