Before he jumped back on the conveyor belt that has carried players in and out of the Celtics locker room this season, Jameer Nelson was a talker.

Two days after the Dec. 18 trade that brought him from Dallas to Boston, Nelson stood up in front of his new teammates and offered to lead. Later, when it became obvious he had no place in Brad Stevens’ plan, the 32-year-old point guard didn’t retreat.

He spoke up about disappointment with his evaporating role, but Nelson also tried to balance out his personal gripes with positive acts. Stevens recalled one video session where Nelson directed the discussion with a group of young teammates.

Nelson spoke to general silence. Phil Pressey and Kelly Olynyk are habitually quiet. James Young is too new. And the fledgling leader of this group — Marcus Smart — is still more of an observer.

But Smart absorbed everything from Nelson that day. The Celtics drafted the rookie point guard based largely on his projection as a leader. He wasn’t as polished or skilled as some players taken later, but no one was tougher, more committed to physically dominating whomever he guarded, or better suited to rallying teammates.

On this day, though, Smart watched. He weighed Nelson’s willingness to share against the veteran’s unpromising situation. There was a lesson to learn just in the way Nelson wanted to help.

“He wasn’t playing, but he’s still in there trying to talk to us, pointing out what we need to do to win games,” said Smart. “Just shows how much he loves the game. Although as a competitor you want to play ­— and I’m sure he wants to play — he’s able to stay focused. It just shows the leadership that he has. That’s why he’s been so long in this league.”

The Celtics finally traded Nelson to Denver for Nate Robinson, whom they immediately bought out. But before Nelson left, Smart filed away another example of how to do something right. He had learned briefly from Rajon Rondo, too.

The conveyor belt keeps rotating, and Smart keeps observing. He also has a rudimentary NBA game to work on, including everything from decision-making to that imperfect jump shot.

“You do have to pay more attention to your own game when you’re a rookie,” he said. “You have to work harder than everybody else when you’re a rookie, and that makes it a little harder. Everybody in this league has worked their whole life to get here, so it’s not impossible. It’s just something you have to do. It’s all on you. I’m working every day, and it’s a matter of time.”

It’s a matter of time, Celtics management hopes, before he learns how to lead veteran players. Stevens, a scholar on the subject, defines an effective leader/athlete as someone who serves his teammates.

“He’s doing a good job of playing well, of handling success and adversity. Those are all precursors to being a leader,” said Stevens. “When you’re a young person you have to earn that kind of respect with your approach. It’s not even your play, it’s your approach.”

Taking their time

Stevens is also worried about the load on Smart. The Celtics coach often expresses caution about not tying the fortunes of this team to a rookie.

“We need Marcus to be a guy who can not only be reliable, but also raise all ships for our team,” said Stevens. “But it’s not fair to put that on him yet.”

Perhaps so, but most Celtics, including the veterans, understand that a coronation is being planned.

“It’s all on him right now,” Gerald Wallace said of the transition from Rondo to Smart. “They just traded their franchise point guard away and drafted Marcus this year. Now he’s the point guard of the future, and it’s a learning process for him. It’s his beginning, and it’s a matter of learning on the job, learning at a fast pace. He has to stay healthy to stay out on the court to compete.”

That means fussing over little things like taping his ankles — something Smart neglected the night he sprained his left ankle last fall — and taking every necessary, mundane precaution.

Since that painful night, there’s rarely a time when Smart isn’t in the trainer’s room. There, Smart, almost routinely, pores over a box score with his left foot planted in a bucket of ice. That may be a nightly treatment for the rest of the season.

Veterans who show the way by taking care of their bodies lead from the trainer’s room as well. Smart is already doing his part there.

“Some of that stuff depends on the personality, how you play, what your expectations of that person are,” Stevens said of what he’s looking for in his next leader — a slow process, considering that he doesn’t plan to at least immediately pick Rondo’s successor as team captain.

Leadership will happen by committee, until someone like Smart is ready to step forward.

“You can be a floor leader and not have the ball all the time,” Stevens said of why he doesn’t need his leader to be a point guard. “(But Smart) has the disposition to be a leader. He has a lot of growth that has to occur because he’s 20 years old.

“There will be days when Marcus Smart is good and days when he’s not, and on the days when he’s not he’s not going to play as much,” he said. “We have plenty of bodies back there behind him that can do the job. We’re going to have to be a team moving forward in the true sense of the word. There’s a lot of evenness, and it will be hard to choose the right player each night.”

Smart’s veteran teammates, though, already believe he has the makeup to become that player.

“I think he does,” said Wallace. “The main thing about him in this league is that he may have it a little too much — he may want to rush back before he can. You’d rather take your time, regardless of how much time there is, so you can compete at 100 percent. You don’t want to go back under 80 percent and have the risk of injuring yourself again.”

Pieces in place

Smart already has something over Rondo at a similar age, maybe even now. The rookie’s confidence isn’t punctured by a bad shooting night. He often shoots better in the fourth quarter, has a knack for making important shots, and doesn’t hesitate to follow up ugly jumpers with a late-game money ball from the corner.

This gumption with the ball stems from natural fearlessness. The afternoon of Dec. 31, Smart wasted no time in jumping off the floor and lunging at Sacramento’s big center, DeMarcus Cousins. Smart had backed into the excitable King with a hard box-out after pushing through a pick. Cousins responded by throwing him to the floor.

Nelson, a peacemaker, immediately jumped in with another lesson — one that temperamental stars like Cousins may never master, and even Smart has already learned the hard way. Smart served a three-game suspension at Oklahoma State last year for going after a Texas Tech season ticket holder named Jeff Orr, who allegedly called him a racial slur.

“What I saw was him about to react, and I didn’t want him to react,” Nelson said of the Cousins incident. “No reason to. There’s no reason to lose money. It was obviously a tough situation because he wanted to react. If I see any of my guys about to get into an altercation, I try to prevent it.”

Rookies will listen to a warning about lost money.

“I’m glad Jameer grabbed me,” said Smart. “He was just kind of telling me, it’s not worth losing money over it, man. You’ve worked too hard to get that money. There’s no reason for you to give it back. Just don’t retaliate. You did what you were supposed to do. You played hard. (Cousins) couldn’t take it, you got under his skin, and you got him off the court just by playing hard. Don’t let that affect you. Just keep playing. (Nelson) was telling me stay away from it, no matter what goes on on the court.”

Right now that may be Smart’s most valuable leadership attribute — that of a good listener.

“When you’re a rookie, a lot of showing respect is doing what (your older teammates) do: showing up early, staying late, playing hard,” said Evan Turner. “At the same time it takes the right type of vet to accept the dynamic that a young player is going to bring to the team.”

Smart appears to be lucky in that way. He’s landed with the right type of vets.

“His maturity level as a player is well above 20-years old,” said Turner. “As far as how he conducts himself on this competition level, he’s not too far out of the equation.

“He’s not selfish either. You play with a lot of guys who only care about themselves, or they’re front runners. Guys who only care about the team when they’re making shots. I’ve rarely ever heard Marcus speak about himself, and that’s the biggest compliment I can give him, because a lot of guys only talk about themselves. He’s genuinely in it for the team.”