DETROIT -- As the game tape rolled in the Detroit Pistons locker room, Boston Celtics rookie Jayson Tatum splashed one long jumper after another through the rim. Watching before he stepped into the shower, Pistons wing Stanley Johnson marveled at how the 19-year-old never misses an open jumper.

Tatum occasionally misfires when he needs to rush his release, Johnson said, or when a defender closes out with particular precision. But Tatum's shots always seem to fall when he can set his feet beneath him properly. The development has been one of the NBA's early-season shockers.

"Anybody who says they're not surprised by his 3-point shooting based on what he did in college is lying," remarked Pistons head coach Stan Van Gundy. "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 (34) 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."

An interesting part about Van Gundy's comments? He still believed in Tatum as much as anybody else before the draft, considering the Celtics' youngster worthy of the No. 1 overall pick even if he couldn't iron out his long-distance shooting stroke right away.

"OK, I'm not surprised (by Tatum's quick success). I thought he was the best prospect in the draft. He's got all the tools," Van Gundy said Sunday before falling to the Celtics, 91-81. "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 (in Boston) for the next however many years."

It's safe to assume a number of other coaches now feel the same way. In short time, Tatum has established himself as one of the NBA's top young players, a foundational piece who could help open the Celtics' championship window for a long time. There was always a lot to like about his game -- the way he can create a shot on command, the way his arms extend all the way to the moon, the way he never seems to waver regardless of the situation.

But the delightfully efficient shooting is what has everyone buzzing. Through the first 28 games of his rookie season, Tatum's 52.3-percent 3-point shooting leads the entire NBA. His first run through the league has been one giant hot streak. He shot 50 percent from behind the arc in October, 48 percent in November, and now sits at 66.7 percent over five December contests.

"(Van Gundy has) probably called me a liar before," said Celtics head coach Brad Stevens, smiling. "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."

Even if they couldn't predict the current level of Tatum's 3-point shooting, the Celtics knew he would need to become comfortable at that range. At Duke, the youngster often lived in the midrange. He had a lot of success during NBA summer league this July, but, again, did so with a lot of tough, contested shots.

"Successful is relative, right?" Stevens said. "You look at the numbers of it. Obviously he can make that shot and we don't want to take that out of his game. That's an important part of the game for him. He should be able to post guys and play off the mid-range some, especially late in the clock, but (if) he shoots 50 percent from there and better than 33 from three, it's better to shoot the 3. And so it's just something everybody knows and we all talk about it. The whole league is shooting a lot of 3s so it's not like it's some grand formula. It just makes sense if you're open to let it fly."

Added Stevens: "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 gonna become that proficient. He still can play in the midrange. We still want him to take good midrange shots. But we have tried to make it an emphasis to not hesitate to shoot. He's so tall that on the catch he can get that shot off and his inclination has probably always been to fake it and drive it. But he shoots it with ease and feels good every time he shoots it."

If there's one complaint about Tatum's progress at this stage, it's that he should be more open to taking risks. Especially when he shares the court with bench lineups, he should be more interested in calling his own number.

But he has benefited from all the attention teams pay teammates Kyrie Irving and Al Horford. Despite Tatum's knockdown shooting, opponents regularly leave him open because they're afraid of the Celtics' two stars. Deep into the fourth quarter Sunday, two Pistons defenders followed Irving when he curled around a weak-side screen. Horford popped to the arc, caught a pass from Marcus Smart, and found Tatum standing all by himself on the right wing because of how hard Detroit scrambled to cover Boston's two best players.

"That's why he was open," Stevens said. "He benefits from that and then he's, obviously, a good player that we think will get a lot better."

Tatum has already shown signs of growth. During the third quarter, he drilled a long 3-pointer when Pistons defender Tobias Harris went underneath a ball screen. The attempt was notable because almost all of Tatum's makes have been catch-and-shoot tries. According to NBA.com, which has only tracked 25 of Boston's 28 games, he has hit 3 of 9 long-distance attempts off the dribble. In those games tracked, the rookie is 0 for 3 on 3-pointers after more than one dribble.

So there's more to unlock. Of course there is. He's just a rookie. It only seems like he's a 10-year veteran when he's delivering in the clutch on a regular basis.

"It's just part of a high-level, high-talented player, an individual," said Irving, no stranger to late-game success. "Some people have it and some people don't. It's just as simple as that. I think he's pretty much shown that he can play -- as people say -- with the big boys at the end of the game."

Tatum's young, but he's doing more than that. He's showing he is already one of the big boys.