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HE IS A rookie starter and much is expected, yet as Jayson Tatum warms up with his teammates before his first NBA game, he appears impervious to the noise and the lights and the pulsating anticipation inside Quicken Loans Arena. This is the home of the Cleveland Cavaliers -- LeBron James' house -- and while Tatum is a teenage visitor, he prides himself on one particular basketball trait: He is rarely jittery on the basketball court.

I've played this game my whole life, Tatum thinks. This is what I do.

He playfully pokes the ball free from fellow Celtic Shane Larkin in the layup line. He launches smooth long-range jumpers in between conversations with his mentor, Kyrie Irving. As the minutes count down before the 2017 tip-off, the fans bellow as the Cavaliers sprint onto the floor. Derrick Rose, the former MVP, leads the charge. Trailing him is Dwyane Wade, the future Hall of Famer. Tatum doesn't intend to gawk, but as he fixes his gaze on them, his mind wanders.

I wore both of their jerseys growing up, Tatum thinks to himself.

He waits, as the crowd does, for his childhood hero to emerge. James jogs out last, his lips pursed, his jaw set. Tatum searches for a hint of recognition, but James looks straight through him.

"He was so much bigger than I thought," Tatum says. "I felt like a kid. I mean, I guess I was one. I was only 19. After that, he was the only thing on my mind. As long as I can remember, he's been the best or one of the best in the league. And that's when it hit me: 'That's LeBron. What am I doing out here?'"

The game unfolds at warp speed. Tatum has an open lane to the hoop and Irving fires the ball into him. Tatum reminds himself to go up strong but not too strong, then elevates with an ease that is initially comforting until ... Ah! Too late! Lying in wait is James, who springs up and swats away Tatum's first NBA offering like it's an annoying gnat.

Tatum's NBA career is exactly 69 seconds old. He's certain this night can't get any worse, but four minutes after that, teammate Gordon Hayward crumples to the floor, his ankle contorted grotesquely out of its socket, his season over.

Immediately, the Celtics need more from the rookie to hang with the Cavs. Immediately, it is too much. Tatum short-arms a layup. Wade blocks another of his shots. Tatum goes 0-for-5 from the floor in that first half, and his team, in shock from the gruesome injury to Hayward, trails by 16 points.

Justin Tatum winces as he witnesses his son falter. Jayson had been coached by his father from an early age to regroup in moments like this. When he was in fifth grade, Justin threw him into high school scrimmages with college-level talent. Jayson would fail -- repeatedly -- but eventually recover, then flourish. On this night, in an NBA forum, Justin believes the result will be the same.

"What I wanted to see," Justin says now, "was for him to fall on his ass, then get back up and do his thing."

Justin's faith is rewarded: His son bounces back with 14 points and 10 rebounds in a 102-99 Celtics loss. Yet the LeBron block gnaws at Jayson. "You set me up," he tells Irving after the game. Irving places his hand firmly on Tatum's shoulder. "You should have dunked it," he says. Tatum nods. He knows Irving is right.

"That," he says, "was the last time I was nervous the rest of the season."

Kyrie Irving has been a mentor to Tatum, calling him a "generational player." Kent Smith/NBAE/Getty Images

THE CELTICS ARE the favorites to win the Eastern Conference this season, and Jayson Tatum's trajectory is a significant reason for that optimism. The Celtics have emerged as a relatively conflict-free collection of rising stars, with Tatum and his uncommon poise representing the epitome of their potential.

The challenge will be how to continue his progression on a team with so many talented veterans who need shots too. Tatum is in a hurry to be great, but he might have to wait his turn on a roster that already features Irving and Hayward.

"He's very composed," teammate Al Horford says. "It's weird to see someone that young pull that off."

He was not born that way. Tatum's unflappable demeanor has been cultivated by two vigilant parents, both college athletes, who raised Jayson separately but who were united in molding their son's future. Yet even they could not have envisioned how quickly his maturation process would need to accelerate. In rapid succession, he was asked to become an integral part of a contending NBA lineup, became a father before his 20th birthday and now has to live up to the sophomore NBA hype that has landed him coveted endorsements that include Gatorade, NBA2K, Fanatics and Beats.

The bull's-eye on his back looms large as Cavs coach Ty Lue declares he has "no ceiling offensively" and Irving claims he can be a "generational player." Tatum knows year two as a one-and-done phenom presents its share of pitfalls; not everyone can pull it off the way Irving, Anthony Davis and Kevin Durant did (just ask Michael Carter-Williams).

Too much too soon?

"No," says his mother, Brandy Cole-Barnes. "He's been working for this."

When Tatum's AAU team, the Spartans, lost in the fourth-grade nationals in New Orleans, his teammates were easily placated with promises of ice cream cones and a swim in the hotel pool. Tatum, still dressed in his sweat-soaked blue and white uniform, dropped to the floor of the Holiday Inn Express, curled into a ball and wept.

"Put on your bathing suit," his mother gently urged him.

"I don't want to go swimming," he wailed. "I want to keep playing."

When Justin Tatum coached at Soldan International Studies High School in St. Louis, he brought his 11-year-old son to mix it up with a team that would go on to win the state championship. His father instructed point guard Pete Sanders, who would later play at Illinois Central College, to pick up Jayson -- slight and not yet 6 feet -- and dog him for 94 feet.

Again. And again. And again.

"Jayson couldn't get the ball past half court," Justin says. "He started to back down."

Sanders stripped the ball from him, knocked him to his knees, shoved him as he tried to navigate his way around the court. Soon Tatum was crying. Sanders fired the ball into the kid's chest. "I'm not letting you give up!" he shouted at him.

"When someone's ripping the ball away from you every time, at some point you just don't want to play no more," Sanders says. "But I kept telling him, 'You better not quit. We don't do quitters here.' When he left, I'd tell him, 'Come back tomorrow and know it's going to be the same thing.' His father told me, 'Don't let up on him.'"

By the time Sanders was a senior, Tatum had sprouted to over 6 feet. He started to slow down and become craftier with the ball. "He figured out, 'Don't let the defense control the speed -- you control the speed,'" Sanders says. "By the time I graduated, I knew he was going to be special."

Tatum enrolled in Chaminade College Prep high school, the decorated alma mater of David Lee and Bradley Beal, battle-tested and unfazed by the storied history of the school. Coach Kelvin Lee set his folding chair along the sideline to watch Tatum's initial preseason pickup game. One minute in, Tatum, isolated on the wing, surveyed the floor, then burst to the left side of the hoop and dunked on a senior captain.

"Yep," Lee says, chuckling, "he made the team."

Tatum was so confident of his standing, he'd angrily confront upperclassmen when they made mistakes on the floor. "You're still a freshman," Lee would bark at his budding star. "You're making mistakes too. Be a good teammate."

By Tatum's sophomore season, Lee had been replaced as coach at Chaminade by Frank Bennett, while Justin Tatum had taken the job at Christian Brothers College High School. Jayson wanted desperately to play for his father, but Missouri regulations required that anyone who transferred from a private school to another within 50 miles sit out a year. Tatum stayed. He wasn't nervous when playing against his father's team, but when Jayson heard his dad barking instructions on how to exploit his shortcomings, he became unglued. "Hearing him on the other sideline trying to defeat me, knowing my weaknesses? I couldn't get past it," Tatum says.

Justin sent multiple defenders at his son, forcing him to become a passer. Jayson finished with a double-double, but Chaminade lost. "It was a huge ordeal," Bennett says, "and when we lost, it felt like we lost everything."

Tatum would play his father seven more times during his high school career and win every game. Justin recalls how Jayson, in his senior season, dropped 46 points on Christian Brothers by the end of the third quarter. In the final frame, as Tatum raced to the hoop in transition, a defender grabbed him, hanging on his shoulder as Tatum slammed the ball through with him in tow. "It was one of those moments when we realized, 'He really can't be stopped,'" Bennett says.

JAYSON TATUM WAS three weeks from finishing his first and only season at Duke, where he had established himself as an NBA lottery pick. Everything was going exactly according to plan -- then his cellphone rang. It was his girlfriend from high school, Toriah Lachell. She was pregnant. Tatum, who had just turned 19, was going to be a father.

"It was a lot," he says, "and it was unexpected."

Tatum told his mother immediately, but he could not bring himself to share the news with his new agent, Jeff Wechsler, or his new coach, Brad Stevens, or with most of his new Celtics teammates after he was taken with the third overall pick. "I felt I had an image of being this clean-cut kid," Tatum says. "I kept thinking, 'How are they going to react to this?' I didn't want anyone to think differently of me."

Complicating matters, he was no longer in a relationship with Lachell, who was still in St. Louis and had planned to attend beauty school. Tatum's aspirations suddenly felt like they were on a collision course with his obligations.

"It rattled him," Cole-Barnes says. "Jayson is a very guarded person. He had just met the Celtics staff, and he wanted to make a good impression. I tried to tell him that it's OK if things don't always go as planned. But he's always been so methodical. He had everything mapped out."

Two months before his son was born, Tatum confided in Irving. "It's a whirlwind of emotions," says Irving, who was 23 when he learned he would be a father. "He was scared. I think most new parents are. But you can't let the judgment of others affect you or infiltrate this incredible blessing."

Jayson Tatum Jr. was born in Boston on Dec. 6, 2017, at 6:30 in the morning. Tatum was present for the birth and played at TD Garden that evening against Dallas.

"All of my worries went out the window when I held my son for the first time," Tatum says. He nicknamed his son Deuce and delighted in his smile. Tatum's mother encouraged Lachell to move to the Boston area, then secured three apartments in the same complex: one for Tatum, one for Lachell and one for Cole-Barnes and her husband. Cole-Barnes would serve as a resident baby sitter while Lachell, who tries to avoid the spotlight (she declined to be interviewed for this story), continued her schooling and Tatum kept playing. "They both have dreams and goals," Cole-Barnes says, "and I want them both to reach them."

Cole-Barnes, who had Jayson when she was 18 and brought him with her to college classes en route to her law degree, made sure her son changed diapers, got up in the middle of the night and learned how to comfort an infant. Lachell, says Tatum, is an attentive, caring parent. He is working hard to be the same.

"Right before All-Star [break], I was very inconsistent," Tatum says. "One day I'd have 20 points, the next day two points. I was dribbling the ball off my leg. I couldn't understand why. The media started asking me, 'Is this because you are a new dad?' I didn't want to blame my son. There are plenty of people in much worse situations and they make it work. I have to do that too."

Tatum elevates for a dramatic dunk on LeBron James in Game 7 of the conference finals. David Butler II/USA TODAY Sports

IT'S MAY 27, before Game 7 of the Eastern Conference finals. Jayson Tatum is taking mental inventory of his veteran teammates. There is Al Horford, headphones on, deep in thought. There is Marcus Smart, blaring music and bouncing off the lockers. Tatum is lounging next to Shane Larkin, who's checking out basketball shoes on the internet.

Out on the court, he doesn't even bother to glance down the other end to see what No. 23 is doing. He met LeBron as a boy, and the photo of the two of them is a treasured possession. But their latest encounter is business.

As the Celtics go through their pregame drills, assistant coach Micah Shrewsbury is asked how his young team is handling the pressure. "I tell you one guy who's ready," says Shrewsbury, pointing at Tatum. "That kid is not nervous at all. He's going to have a great game."

Tatum leads Boston with 24 points. With 6:46 left in the fourth quarter, he gathers the ball above the foul circle. Horford sets a screen and Tatum takes a strong dribble to the left. I'm going to go hard to the basket, maybe try to dunk it, draw some contact, Tatum thinks to himself. He turns the corner, cuts hard to the hoop and ... there he is: LeBron James, all 6-foot-8, 250 pounds of him, sliding over to help.

"I wanted to dunk it as fast as I could because LeBron is so great at blocking shots and timing the ball," Tatum says now.

He elevates over James and ferociously slams it through. The Cleveland players stand transfixed, stunned. Tatum turns and chest-bumps his boyhood idol.

"I don't know why I did that," Tatum says. "I meant no disrespect. It was all emotion, you know?"

His thunderous dunk cuts Cleveland's lead to two. Next time down, he drills a clutch 3, and for a moment as TD Garden erupts, he allows his imagination to run wild.

"I started thinking, 'We're going to the Finals,'" Tatum says. "Only we didn't."

The Cavs win 87-79 and eliminate Boston. Tatum spends his summer working on finishing around the basket, first with Penny Hardaway, then with Kobe Bryant and later on a working vacation with Irving in the Bahamas, where he finds himself paired daily opposite Durant. "The two of them spent the whole week jawing at each other," Irving says. "Fantastic."

Deuce is healthy and growing. He has two teeth now, and he's standing up, teetering around the furniture with an unsteady gait. He loves bright colors, "The Lion King" and sleeping on his daddy's chest. Cole-Barnes scolds her son that Deuce needs to learn to sleep in his own space. Two weeks later, a photo appears on Instagram of Tatum in his son's crib, his long legs dangling over the side, with Deuce cuddled up next to him.

Tatum has grown so much in one NBA season yet knows he has miles to go, on and off the floor. "Sometimes when I'm in my room," he says, "I still have moments where I'm 20. I've probably watched [the dunk on LeBron] a million times."

He knows that it's winning, not highlights, that gets you places. His life is hurtling along faster than he imagined, but the kid insists he can handle it.

"I'm not afraid," Tatum declares, "of anything."