DALLAS — Dirk Nowitzki occasionally finds himself reminiscing about his rookie year, now that the Dallas Mavericks are relying heavily on a kid who was in diapers when Nowitzki made his NBA debut in 1998.

Few of those memories are fond for Nowitzki. It’s easy to forget now that his legacy is secure, but Nowitzki looked like a bust a few months into his career. He felt like one, too, and wondered if he belonged in the NBA or was better off returning home to Wurzburg, Germany.

Dennis Smith Jr., a 20-year-old rookie who hopes to eventually take the torch from Nowitzki as the face of the Mavs’ franchise, doesn’t have similar doubts about his game.

Smith, a point guard with pogo-stick hops who, like Nowitzki, was drafted with the No. 9 overall pick, is quite certain he’ll be a star in this league. He’s brash enough to try to dunk on Draymond Green and laugh in his face when the reigning Defensive Player of the Year attempted to intimidate the rookie by following up a highlight-halting hard foul with a little welcome-to-the-NBA trash talk.

“He’s fun,” Nowitzki says of Smith, entertained by the precocious pup’s rare athleticism and intrigued by his potential.

Of course, it hasn’t been all fun and games for Smith. It rarely is for rookie point guards, particularly starters who are teens when they make their NBA debuts. For every highlight there’s a handful of film clips the coaches use as teaching moments.

The Mavs are in the early stages of a transition period — a nice phrase for “rebuilding” — simultaneously dealing with Nowitzki’s decline while prioritizing Smith’s development, with 25-year-old Harrison Barnes bridging the gap as Dallas’ go-to guy. The process will likely result in a high lottery pick to pair with Smith in the future, as the Mavs sit last in the Western Conference standings with a 9-25 record.

The hope had been to allow Nowitzki to serve as a complementary threat on a contender during his twilight. That didn’t happen. The Mavs swung and missed on several superstar targets after stripping down the aging roster of their 2011 title team, and Nowitzki, 39, has accepted the reality of rebuilding and his role as a mentor.

Nowitzki knows it’s unlikely that he will get to experience the payoff from Smith’s growing pains.

“That’s part of the league. You can’t play forever,” said Nowitzki, who would like to extend his career at least one more season, if his body is willing. “I’m sure some of the veterans that we had back then were saying the same thing back when I was 20. My rookie year was way, way worse than his rookie campaign. I mean, it was tough. It’s just the cycle of the league. New guys come in, older guys go out. That’s just part of it.

“Really, the last couple years, ever since we won the championship, have been a challenge here, but it’s a challenge that franchises go through. If you’re the Spurs, I guess you can [keep winning]. Other than that, all the other teams go through ups and downs. We’ve been great for a long, long time. Now it’s our time to go through some struggles and some growing pains and hopefully get it back on track sooner or later.”

“He just turned 20. It’s crazy, sometimes you have to tell yourself how young he is. The sky’s the limit.”

Mavericks star Dirk Nowitzki, on rookie Dennis Smith Jr.

Dallas has been competitive since digging a deep hole with its 2-14 start, but the Mavs’ improvement is much more attributable to the performance of veterans than the progress of Smith.

The Mavs’ best lineup by far has been Nowitzki with bench players, particularly guards J.J. Barea and Devin Harris, who both have years of experience working with the big German. Dallas is plus-123 in 265 minutes with that trio on the floor, according to NBA.com stats.

“We’ve known each other for a long, long time,” Nowitzki said. “It’s just a smart group.”

The getting-to-know-you process with Smith, who has missed eight games due to injuries, hasn’t been as smooth. The Mavs are minus-107 in 389 minutes with Smith and Nowitzki playing together. Those numbers are skewed negatively by the fact that those stretches typically come against opposing starters, but Smith might be able to learn a lot by watching the Mavs’ old guards, especially Barea, a sub-6-footer who wasn’t blessed with anything close to Smith’s explosiveness but has carved out a productive, 12-year NBA career.

Smith, who was used to simply soaring over defenders by virtue of his 48-inch vertical, has learned the hard way how difficult it is to finish in NBA traffic and how dangerous it is to try to dunk everything. Smith is shooting only 51.0 percent in the restricted area, per NBA.com, far below league average (57.8 percent), exposing his need to develop a floater and the ability to finish with his left hand.

“We’ve got to be more mindful of the big picture,” coach Rick Carlisle said. “It’s an education process for him. You’ve got to show him the things that he needs to do to increase percentages, stay out of harm’s way, involve his teammates.”

Smith cites adapting his game to the Mavs’ system, which emphasizes ball movement, as his biggest challenge to this point. Carlisle has harped on Smith about over-dribbling, a habit formed by being counted on to create every possession for his high school, AAU and North Carolina State teams.

“It’s a systematic team, so I’m trying to convert my game into buying into the system,” said Smith, who is averaging 13.8 points and 4.2 assists per game but shooting less than 40 percent from the floor. “Just listen and watch the film. Really, whatever they say, that’s what I’ve got to do.”

How can Smith do that?

“I’m not sure yet,” Smith said. “That’s something I’m still working on.”

So is his coach, as Carlisle acknowledges the need to evolve the offense to feature Smith.

“It’s a two-way street: We have to adjust to him in a certain way, and he has to adjust to the rest of the team,” Carlisle said. “When you’ve got a guy that can do these kinds of things with the ball, he has to have the ball to do what he does best in many cases. But learning to play off the ball and learning to play without the ball is another important part of learning to play the entire game, and also learning what the other positions on the floor are like as a point guard.

“Coming into this, his learning curve probably couldn’t have been more steep with everything that’s going to be coming at a young point guard that’s going to be projected and then put into the starting lineup. But each week, he’s getting better and better.”

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Dennis Smith cruises by Pau Gasol and throws down a massive two-handed dunk.

Nowitzki’s presence gives Smith a model for how a franchise centerpiece should conduct himself. Smith sees how hard Nowitzki works and vows to hold himself to that standard. He notices that Nowitzki has strong relationships with support staff members and treats everyone with respect, and Smith plans to be the same kind of professional.

“It’s just about being a good person,” Smith said. “If you have the talent on the court, it’s easy. It’s about what you do off the court to be the face of the franchise.”

Nowitzki has intentionally not been too involved with Smith’s day-to-day development. The rookie has near-daily film sessions with Carlisle and assistant coach Jamahl Mosley. Player development coach God Shammgod is constantly working with Smith on his skills. Wesley Matthews and Harris have made a point to take Smith under their wings.

Nowitzki is always willing to help and has a friendly relationship with the rookie. But Nowitzki tries to choose his words of advice wisely, not wanting to overwhelm Smith.

“As a rookie, you have a lot of people in your ear,” Nowitzki said. “I don’t want to harp on him even more, but if I see something that I think can help him, especially during games, I go up to him. Other than that, I let him play. I let him be, let him play his game.

“He’s a competitor. He wants to win. He wants to learn. He just turned 20. It’s crazy, sometimes you have to tell yourself how young he is. The sky’s the limit. If he keeps working, he’s going to have a long, great career.”