As the rookie wall approached, Jaylen Brown pushed through. It’s rare to see a newbie playing such a prominent role this deep into the season, but here he was. Game 5 of the Eastern Conference Finals, the Boston Celtics getting thrashed by the Cleveland Cavaliers in game 95 of his rookie year.

Their season was falling just out of reach, but Brown’s career was still just beginning. With Kyrie Irving waiting for a charge and Lebron James breathing down his neck, Brown did not slow down. He gained speed, slipped by the charge and hit a graceful lefty finger roll while holding off James.

The moment was a flashback to the first time Brown met James on the court, when he drove right on LeBron, walled him off with the left shoulder and threw it down. Brown had 19 points that night, just his fifth game in the NBA as a newly minted 20-year-old, but he was clearly mature beyond his years.

That was the theme with Brown all year. He was 20 going on 30. Even when he didn’t look ready for the speed or power of the moment, he clearly belonged. His emotional and physical maturity was what took him from being a project in June to a rotation stalwart in late May.

He broke in his NBA career blowing past LeBron and closed his rookie season getting beat by him. In the months between, Jaylen emerged from a flat-top flyer to a versatile multi-tool swing, playing three positions with skill, awareness and rhythm.

Brown’s biggest development came without the ball, learning how to play NBA defense in an impressively brief time. This is typically the biggest challenge for any rookie, coming from college where zone is eschewed for complicated pick-and-roll schemes. By the second half of the season, Brown had a good feel for footwork on ball and understanding how to take contact without fouling. But the complications of playing man defense and dealing with help and switching was always a challenge. When he would miss a weak-side action or blow a switch, coach Brad Stevens was often honing in on the mistake.

“I had to fight for everything I got,” Brown told the Boston Globe. “I had to earn every minute, earn every possession, and I liked it like that.”

Stevens often pulled Brown after a mistake or two on either end, typically more focused on gameplan or read mistakes than forcing a play or missing a shot.

“If you’re not playing hard, you’re not playing,” Danny Ainge told the Globe. “And when you make mistakes, you come out of the game and you’ve got to know what you’re supposed to be doing, and you’re held to the same standards as veteran players.”

Stevens seems to hold his rookies to an even higher standard, giving Terry Rozier a short leash last year and an even shorter one for Brown this year. But despite some stumbles out of the gate, Brown eventually found his offensive rhythm. He was a force in transition and had a great sense for when to cut back door for alley oops. He was in many ways the player the Celtics had always dreamt of in Jeff Green, but will require significant refinement to get there.

He has some clear areas of improvement, yet Brown finished the season as second in points per possession among rookies at 0.949, just two thousandths of a point behind Joel Embiid’s incredible short season. While he ranked 134th out of 223 players with at least 500 possessions per Synergy Sports, he was apparently quite good for a rookie. Brown holds himself to a higher standard — something that endeared him to Ainge’s front office when they drafted him — and there are a few things he is already working on this summer to get into the upper echelon soon.

“He was physically and emotionally older than his age. I feel it helps him a ton,” Ainge told the Boston Herald. “He just had to learn an awful lot about the game. It helped him defensively and offensively. He handled a 100-game season in great fashion. He really played well.”

His first objective is to become a more controlled dribbler. He is already releasing videos of his dribbling and shooting drills, focusing on erasing his bad dribble turnovers that plagued him throughout the year. His crossover and behind-the-back dribble often ran away from him, but he was always persistent.

He is learning how to drive through traffic using more than his speed and height, developing hesitation moves and making his dribble counters more compact and low. His dribble was way too high, allowing for easy lost balls or more of a chance for the defense to poke it away. The best ball handlers get close to the ground to make their moves shorter, quicker, and a lower bounce to minimize ball exposure. His teammate Isaiah Thomas barely lets the ball leaves the floor, making him one of the hardest players to swipe in the league. It’s something that Avery Bradley has made tremendous progress on over the past three years, allowing him to become a better playmaker and scorer off the bounce.

When he did try to dribble low at the point of contact, he struggled controlling the ball in the same hand for multiple dribbles. Initiating contact while dribbling with your off hand is something that usually looks easy, but requires seamless coordination. He is comfortable using his left shoulder to wall off while dribbling with his right, but defenses will shade him left until he can do it consistently with both hands. Brown will need to continue to work on his physical dribbling game to really become a devastating driver and minimize these mistakes. Once he does, he’s going to be pretty unstoppable getting in the lane.

A better dribble will open up an already savvy perimeter game. Brown works well off the ball and knows his spots. He attacks closeouts at a veteran level with controlled pace. But he needs to work on his vision so he can advance the ball and keep the play flowing. This isn’t a glaring weakness, but rather a natural evolution for any rookie. It’s his post-up game where this can really shine.

Brown was already a successful post-up swing as a rookie, taking advantage of mismatches to lead all rookies in points per possession out of the post-up at 0.976, per Synergy Sports. One of the tenets of Boston’s second unit offense was to find post-up mismatches to function as a passing fulcrum, with which Marcus Smart excelled. Brown proved to be a good back-to-the-basket scorer out of these situations, translating his college skills to show off good footwork and physicality. Jaylen will work on his passing reads over his shoulder next year to find weak side shooters and function much in the same way as Smart did during the regular season. With his high reach, he could end up being a deadly creator out of the post, especially when he’s going against guards and wings.

He welcomed the challenge of everything thrown at him this year, but he knows he is committed to rapidly building up his skill set to become a franchise cornerstone as soon as possible.

“I don’t think people expected me to contribute as much as I did, and now getting to the Eastern Conference finals and losing, it builds a hunger,” Brown told the Globe. “I have a bad taste in my mouth, so I’ve got to put in work in the offseason and come back even stronger.”

He got off to a great start this year and is pushing through for an even better one next year. What once looked like a reach and a project a year ago is looking more like a sure thing every day.