Before they were NBA stars the Wizards’ Bradley Beal was a mentor to a young Jayson Tatum. It allowed Tatum to become the next Celtics star.

During the 2011-12 season, Rajon Rondo was emerging as a superstar and the Boston Celtics were making a run at LeBron James and the Miami Heat. They would end up losing in the Conference Finals after a historic performance from James that catapulted him to his first of many titles. A year later, they would be disbanded and Brad Stevens would usher in one of the swiftest rebuilds ever seen.

As one elite team fell and another rose in its place, a 13-year-old Jayson Tatum was getting up shots in a gym at Chaminade College Preparatory School in St. Louis. The eighth-grader was following in the footsteps of his unofficial big brother Bradley Beal, who had just scored 32.5 points per game for Chaminade’s varsity team as he won Gatorade National Player of the Year. One basketball prodigy was heading to University of Florida while the other was still trying to figure out what type of basketball player he could become.

Jayson’s father, Justin, was back from a pro basketball career overseas and they needed more focused direction and training for the junior Tatum. They asked Beal what to do and he put them in touch with his trainer Drew Hanlen, the man who Beal had been working with since he was 13. Hanlen had just finished second in the NCAA in three-point shooting and was building up a professional clientele of trainees that included former Chaminade star David Lee and other elite prospects like Andrew Wiggins, Zach LaVine and Kelly Oubre.

Tatum’s mother Brandy called Hanlen when Tatum was still in eighth grade, insisting he had special potential and needed Hanlen’s guidance to put it all together. Hanlen resisted, saying that he didn’t really work with eighth graders. When Tatum became a freshman at Chaminade, she tried again and again and again. Her persistence was breaking through and Hanlen admired the family’s dedication.

Then a call came in from Beal.

“Hey bro, is there any way you could work out Jayson? He’s like a little brother to me,” Beal told Hanlen.

The next thing he knew, Hanlen was meeting with Tatum at Chaminade every morning at 6 a.m. for a workout. Beal was his trial run to build a great player.

Tatum was going to be his masterpiece.

“Brad and I joke with Jayson and tell him that he had it good, because when I was working out Brad, I was first starting my career,” Hanlen told Celtics Wire. “So there was a lot of trial and error. But by the time I started working with Jayson, I had a strategic plan for his development and there were different phases of his development we wanted to attack each and every year that were going to ultimately prepare him to be ready to contribute right away in the NBA.

“I think that’s a big reason why he’s been able to step in and provide value to the Celtics right away.”

The basketball world has been shocked by Tatum’s rapid acclimation in the NBA. He has been adjusting since day one and is thriving as the pressure ratchets up on him. But those who have worked with him his whole life are less shocked and more encouraged that he’s continuing to advance at the same rate in the pros as he did back in high school.

“The crazy thing is, I wasn’t impressed because I always see it; I almost call out his moves,” Beal said after facing Tatum in an official game for the first time on Christmas. “But it’s amazing to see him do it on this level. He’s going to be a special talent, a star in this league for many years, for sure.”

As Tatum was quickly growing and learning the game as a point forward in high school, Beal saw the template of a future NBA player being penciled in. He identified the potential for Tatum to be a major NBA talent early.

“Middle of high school, man – maybe even before high school because he was 6-foot-7 [or] 6-foot-9 in high school,” Beal said. “And he was number one in his class, so like, that was kind of a no-brainer. I seen the potential early. We all seen it. I’m just happy that he was able to stay level-headed and finally let his dream come true.”

Tatum was the only player Hanlen worked out for the entirety of his high school career. He would take on players like Beal, Oubre and Wiggins either as they were transitioning to college or to the pros. He’s become one of the league’s go-to trainers for the draft transition and as players hone skills mid-career. But Tatum was the exception for a reason.

He had a special maturity to him, even as he was entering high school. His father always had him competing against older players or players from other cities in AAU, trying to push his limits instead of dominating his peers. So when Tatum was afforded the opportunity to work with players entering the NBA, he was able to fit right in.

“He was in a group chat with myself, David Lee and Bradley Beal since he was 13 years old,” Hanlen said. “He was doing workouts while he was in high school with Jordan Clarkson, Andrew Wiggins and Kelly Oubre. That helps you develop an NBA mindset. Also, he’s able to bounce ideas. That’s why I think he has such a good maturity.”

Hanlen had worked all the kinks out of his developmental program with Beal. He knew how to help established professionals enhance components of their game, especially shooting. Beal was an all-encompassing project, which turned out to be a great success. But it was still a beta program until subject 2.0 came along.

“I think Jay was far more advanced than I was for sure, because I started late in high school,” Beal said. “We were starting with Jay at the beginning of high school. So I think it helps us out a lot because I was kind of a guinea pig. And Drew was just learning the league’s game because he was in college before. But he was learning the league’s game, helping me out as best as he could. And even to this day he still helps me, shaping me into who I am today.

“But Jayson, he’s going to be really special for years to come. I think that definitely gives you a little bit of a jump-start, especially when you have a few years under your belt to know what the league’s like, knowing how guys are playing, knowing what your role will potentially be. It’s picture-perfect, for sure.”

Tatum had been playing against high school kids since he was 9 years old. His father Justin started coaching high school in 2007 and would have Jayson take on his best player, a guard who was ranked toward the top in the state of Missouri. Whenever the Tatums felt like Jayson was getting comfortable with the competition, they would seek a bigger foe to take down.

“Jayson has always been challenged to play a couple of levels up,” his father told Celtics Wire. “If the city of St. Louis wasn’t enough of a challenge for him, we took him down to Memphis to play against players that were better than him or futures that were better than him. We would play against them or join them. We wanted to see what they had to offer.”

Justin never let Jayson waltz into being a star player. He had just spent a career overseas and knew what he had to do to make sure Jayson would dominate at the highest level. It was more than just making him play guys his size; it was also creating a mindset in the daily challenge and work to keep his bar for success as high as possible and chase it without hesitancy.

“I’m very aggressive. Sometimes I’ll forget that that’s my kid, but he’s gotten used to it,” Justin said. “He’s like, ‘My dad broke me and put me together to the point that any coach in America cannot be tougher or push me harder than him. I might as well be able to bounce back.’ That’s why I love how Brad [Stevens] stays on him and with Coach K, he’s getting some of the best coaches in the world after high school.

“As a dad, that’s the thing that I love the most to see, because he’s not somebody being told to do something right away. They need him to do what they want him to do to help the team and it changes his mindset and it’s challenging him in different ways. That’s a lot of what I’m seeing of the growth in his game. He is probably not so phased with things right now because he knows – and I know it and his mom knew it – that he expects to make those plays and fight through that adversity.”

This mindset was one the keys to his rapid and persistent growth that Hanlen identified early.

“He’s a really good kid that’s been raised the right way and he hasn’t had any distractions,” Hanlen said. “Just a mindset and a maturity that off the court, he handled himself in a good way without many distractions and it’s been all about being a better basketball player his entire life.

“He put in the right work – work that directly translated to the NBA game.”

The tireless work, the dedication and doing it the right way. These were all the factors that allowed him to ascend to not just a top pick, but part of the core of a team sitting on top of the East.

“It’s amazing to see his growth, man,” Beal said. “I’ve been watching him since he was a pup. Now he’s a man finding his way into the league.”

He’s still a teenager, but he’s playing like the grown men he has been working with and competing against his whole life. As trial and error from those who blazed the trail before him cleared the way, Tatum has spread his wings and taken off at full speed.

For those closest to him, the question has never been about how wide he could spread those wings, but about how far he can soar. As they look out to the horizon, there doesn’t seem to be a limit in sight.