He was on the floor when it happened, but he didn’t see it. Not at first. “But then I looked back,” Jayson Tatum said, “and I seen him point at his leg. And I thought, ‘Oh my God.’”

As he laced up his sneakers last week at UCLA’s old practice gym ahead of a game against the Lakers, the 19-year-old Celtics rookie recalled one of his first moments as a professional player, a terrible and unfortunate event that changed a lot of things for a lot of people. Since then, the Celtics have leaned on Tatum quite a bit. Maybe that was always the plan. Maybe his outsize talent would have demanded attention and playing time all along. But it’s hard not to consider what happened to Gordon Hayward in the first quarter of his first game as a Celtic and think that the ripple effects accelerated Tatum’s timeline.

“He was over there,” Tatum said, reenacting the fateful play in which Hayward fractured his left tibia and dislocated his ankle in a brutal fall against the Cavs, “and I was over there,” he continued, motioning to the other side of the floor. Tatum was shocked and sad for Hayward. Months later, he still can’t believe it happened. But the effect on his own role took far less time to process.

“Instantly,” Tatum said. “It was a dramatic thing to happen. It was awful. In the back of my mind, I kind of understood that, not just for me but for everybody, everybody is going to be expected to do more.”

For the first three months of the season, he did more. A lot more—or at least more than almost any honest, non-kelly-green-loving fanboy would have predicted for a kid who played one college campaign before leaping to the pros. Between October and December, Tatum shot 50.8 percent from the floor, 47.1 percent from 3-point range, and 82.4 percent from the free throw line. Through it all, his offensive rating ticked steadily upward, rising from 116 in October, to 118 in November, to an eye-popping 121 in December, per Basketball-Reference.

But while climbing so high so quickly made a lot of people notice—as expected, Tatum was named to the Rising Stars game, for rookie and sophomore players, at All-Star Weekend—it also saddled Tatum with more responsibilities. Before he knew it, he went from an understudy with potential to a starring role in Boston’s big-budget production. It isn’t easy for anyone to fill in for a max-contract player, least of all someone in his first NBA season. In that regard, Tatum has so far delivered his lines expertly, minus a few stumbles in January. In 11 games this month, his production dipped a bit. He has a 96 offensive rating, and he’s shooting 41.3 percent from the floor and a league-average 35.1 percent from 3-point range. In other words, he’s more recently looked like a pretty good rookie instead of a veteran All-Star. (For the season, he’s still shooting 44.3 percent from 3, third-best in the NBA behind Klay Thompson and George Hill, though it should be noted Thompson averages more than twice as many attempts as both of them.)

After a smooth start, the past few weeks have provided some tests for Tatum and the Celtics (they’re 5-5 over their past 10 games heading into into Wednesday’s national TV matchup with the Knicks). While Boston stumbled through its longest losing streak of the season—a four-game skid in mid-January that included ugly efforts against the Pelicans, Sixers, Magic, and Lakers—Tatum tried to snap himself out of personal funk. He went a combined 8-for-22 against Philadelphia and Orlando, and he was all but invisible against the Lakers (he finished with four points and didn’t make his first basket until the fourth quarter).

Predictably, some media members (and even hard-core Celtics fans) wondered whether Tatum had crashed into the proverbial rookie wall. Perhaps he did. Tatum played 29 games at Duke last season; he’s already appeared in 51 for the Celtics. But if the professional grind has slowed what began as a blur of forward momentum, Tatum would hardly be the first rookie to fall off the pace midway through his inaugural marathon. It happens. It’s normal. How Tatum handles it and how quickly he regains his wind is more important.

For his part, Tatum does not lack for belief in his ability. When we spoke last week, he seemed unfazed by his recent struggles. In the midst of that shooting slump, he told me he came into the NBA knowing what he could do and what he could offer the Celtics. Tatum looked better in a one-point win at Denver this week. He was all over the box score: 20 points on 8-for-13 shooting (including 2-for-4 from 3), six rebounds, four assists, one steal, one block, and one poor defender named Torrey Craig put on skates for our amusement.

Jayson Tatum just spun Torrey Craig back to the G-League pic.twitter.com/tVt8heANtq — James Wobden (@World_Wide_Wob) January 30, 2018

Even so, there is a sizable gap between thinking—or even “knowing”—what you can do and then repeatedly proving it in the NBA against hypertalented professionals several years your senior. Consistency is the difference between being in the Rising Star game and being an actual rising star in the league. In that context, his difficulties in January weren’t nearly as surprising as his blistering three-month start.

“I wasn’t as surprised as everybody else,” Tatum said.

Along with the skill set, you can imagine that kind of self-assurance was part of what attracted Celtics general manager Danny Ainge to the 6-foot-8, 205-pound forward in the first place. After all, Ainge is widely regarded as one of the more confident executives in the league, and he repeatedly insisted post-draft that Tatum was his guy all along and he would have taken him first overall had the Celtics not made a trade with the Sixers and moved back to third. Whether anyone buys that or dismisses it as more spin from Dealer Danny doesn’t matter much to Tatum—because Tatum believes it. He said he jokes with Ainge all the time about how the GM should have just stayed put and taken him with the first pick.

“I could have gotten paid more,” Tatum said with a laugh. “No. 1 sounds better than no. 3, too.”

Whatever on-court adversity Tatum has experienced of late is minor compared to the tragicomedy of the still-unfolding Markelle Fultz saga. Because the two were key components in the deal that sent Fultz to Philly and Tatum to Boston, they’ll forever be linked in their own hypothetical what-if game that we sometimes play when imagining alternate NBA universes.

So far, their NBA experiences and prospective career arcs couldn’t be more different. Tatum has had three good months and one that’s been less memorable; Fultz played four games, got shut down because of a mysterious shoulder “injury,” and now shows up only in shaky iPhone videos (including a pretty impressive backward shot). Tatum has thought about his draft classmate quite a bit. He said he knows Fultz “pretty well,” called him “a great player,” and predicted “he’ll figure it out.” But Tatum also shook his head and puzzled over the circumstances just like everyone else.

“I don’t exactly know what happened,” Tatum said. “I think it’s kind of unfortunate. I’ve known Markelle for a while. We’re friends—even before we got drafted. I hate to see what he’s going through. Hopefully whatever is wrong gets fixed. We all have things that need fixing.”

It was as close as Tatum came to hinting that he needed to make some adjustments to his game, but he did reveal that Brad Stevens reminds him of his imperfections on a regular basis in practice and film sessions. In Stevens’s parlance, there are “teachable moments” for all of his players (the head coach insisted he points out the mistakes of Kyrie Irving and Al Horford as much as he calls out the younger guys). The idea, Stevens said, is to “find what makes each guy tick” to “add value to winning.” If that sounds a bit TED talk–ish, Tatum provided the colloquial translation: “He’s on me a lot.”

Tatum said that doesn’t bother him. He played for Coach K. “Coach K is a yeller,” he said. “Coach K yelled at me all the time. He yells at everyone.” Stevens doesn’t yell. So what does he get on Tatum about?

“Everything,” Tatum replied.

Any fun stories about that?

“Fun?” Tatum asked, seizing on the word. “Those moments aren’t fun. But I understand when he gets on me and why he singles me out. He’s not trying to be mean. He’s trying to get me to the next level.”

On his end, Stevens credited Tatum for how he handles the feedback. “He lets me coach him and coach him hard,” Stevens said. But for all those teachable moments in the pursuit of adding value to winning, it was the time that Stevens didn’t have to coach Tatum at all that sold him on his new rookie.

Stevens believed they had something special when they drafted Tatum, but he said he knew it for sure during the preseason—and it wasn’t a monster dunk or a deep 3 in a crucial situation that convinced him, either.

In early October, the Celtics played an exhibition game against Charlotte. The Hornets were running offensive actions that the Celtics didn’t break down with their players beforehand, because it was the preseason, and that’s how these things go. Stevens recalled that Tatum got beat defensively on one of those actions. “But the next one,” Stevens said, “he adjusted. He did it on the fly, on his own. That showed me (a) the mentality that he’s competitive and (b) he’s got to solve this puzzle quickly, and he did.”

That’s the kind of thing coaches in general, and Stevens specifically, love. As currying favor goes, it also didn’t hurt that Tatum shot well at a time when the Celtics desperately needed to add offense. Stevens took umbrage with the notion that Tatum’s role changed because of Hayward’s injury—he pointed out that Tatum was already in the starting lineup—but he conceded that there were more minutes to gobble up after that, especially with Marcus Morris out to start the season.

“He’s a guy who has a knack for putting the ball in the basket,” Stevens said about Tatum. “It doesn’t surprise us. Obviously, his [3-point] percentage is up there at the top of the league. I don’t know that anyone would have predicted that right out of college.”

Not even Tatum. After he told me he knew what he could do all along and that he expected to hit the NBA floor in full sprint, he relented a little when I pressed him on his otherwise uncrackable confidence.

“I was surprised by how well I shot the ball,” Tatum said. “I’m not gonna lie. That’s the thing that probably surprised me. In college, I only shot [34.2] percent from 3. Now I’m [third] in the NBA in 3-point percentage. I didn’t expect that.”

It’s why he spent all those hours hoisting up shots in the offseason. Tatum forced himself to make 250 3s every day for seven straight weeks before the draft—first at Duke, then in his hometown of St. Louis, then in a small gym in Los Angeles by LAX. “Some of those were long days,” he said.

They still are. When he’s not breaking down his shot selection with assistant coach Micah Shrewsberry, Tatum runs questions past Kyrie Irving and Marcus Morris, two vets with whom he’s become particularly close. He and Irving share the same agent and a natural Duke connection—they recently gushed about the Blue Devils’ new recruiting class—and he sits across the aisle from Morris on team flights. Al Horford called it “impressive” how quickly Tatum “understood finding his shots on the court,” and Irving described the rookie’s ability to get to open spots for jumpers and drives as “uncanny for a young player.”

By all accounts, Tatum’s assimilation into the Celtics system has gone to plan. The net effect is that a kid who is still more than a month removed from his 20th birthday has become an important part of a team that leads the Eastern Conference and has designs on a deep playoff push. And yet there are varying internal interpretations about his season to date.

While we talked about Fultz, the conversation drifted to some of the other rookies. To Tatum’s mind, he’s in a unique situation relative to most of the rest of his class. Where other rookies might be free to stuff stat sheets because, as he put it, they’re “the focal point” on mid-to-low-level teams, Tatum is required to play his part for the good of Boston’s collective aspirations.

That made sense to me—until I talked to some people familiar with the Celtics’ thinking. One person called the notion that Tatum has been restricted by the system a “false narrative,” while another flat out laughed. I didn’t understand why until I asked Stevens what Tatum needs to work on most. He explained that because Tatum is talented and draws attention from opposing defenses, it “opens up opportunities” for Tatum to get others involved. Basically, because Tatum “sees the floor so well” and is “very bright,” Stevens believes he can raise his game by elevating others. If that sounds like a next-level ask of a first-year player, that’s because it is.

“You don’t expect less because they’re young,” Stevens said.

Put another way, where Tatum is concerned, they want even more.

While the Celtics push Tatum forward, he compares pretty favorably to his peers. There have been moments this season when he’s been the best player on the floor—like when he dunked on pretty much all of the Pacers. Former Celtic Brian Scalabrine, now a team broadcaster for NBC Sports Boston, calls those “five-minute flashes.” “He puts together five minutes that aren’t just good for rookies,” Scalabrine said. “They’d be good for KD.”

That’s a bit much, but there’s a reason there’s been so much buzz about Tatum and why he’s consistently appeared near the top of various rookie rankings. Among first-year players, in addition to being tops in 3-point percentage, he’s: fifth in free throw percentage; sixth in points, rebounds, steals, and blocks per game; and eighth in field goal percentage. As a result, he’s gotten a lot of attention (though some of it has been unintentionally hilarious; during the Clippers game, ESPN analyst Mark Jackson pumped him up as a Rookie of the Year candidate at the exact moment Tatum tossed up an air ball).

“Our class is pretty strong,” Tatum said. “We have a lot of talent. A lot of guys who maybe guys didn’t expect to be that good. Donovan Mitchell. There’s a lot of players. Jordan Bell. [Kyle] Kuzma. I really like that kid Mark Lauri.”

Lauri Markkanen?

“Yeah, from Chicago,” Tatum said. “I knew he was gonna be good. I always enjoy watching him play.”

Tatum said he watches basketball all the time and follows the rest of the rookies closely—which made me wonder who he thinks should be Rookie of the Year. He dodged the question at first before allowing that he’s thought about it. “Me, Ben [Simmons], Donovan Mitchell, Kuzma—those are the names I hear the most for Rookie of the Year,” Tatum said.

As he put it, the award is “more of a stat-based thing,” which is why he initially said he doesn’t think he’ll win. Then he reconsidered and thought about it for a moment, and that familiar confidence peeked out from behind the facade.

“I mean, if we’re going to the championship …” he said, trailing off with a slight smirk on his face. He didn’t finish the thought. He didn’t have to.