Jayson Tatum is emerging as Kyrie Irving's fourth-quarter sidekick

BOSTON -- Crunch time is not typically a spot for rookies but, well, Jayson Tatum hasn't been your typical rookie.

With the Celtics clinging to a one-possession lead with under a minute to play in Saturday's game against the visiting Phoenix Suns, the 19-year-old Tatum turned the corner after Al Horford slowed his defender with a screen on the right wing. When no one stepped up to deter Tatum, the rookie exploded to the basket for a two-handed slam.

Even though the Celtics boast one of the NBA's best clutch-time players in Kyrie Irving, Tatum has distinguished himself with a final-quarter poise that's well beyond his years. Saturday's late-game slam helped Boston hang on for a 116-111 triumph over the Suns at TD Garden.

Tatum is averaging 4.4 points in the fourth quarter this season, a mark that's second on the Celtics behind only Irving, who is fourth best in the NBA at an average of 7 points in the final frame.

Narrow to clutch time -- the score within 5 points in the final five minutes -- and Tatum's exploits are even more remarkable. After Saturday's win, he was tied for ninth in the NBA in clutch points (33) while shooting 63.2 percent, the highest rate among the top 20 in clutch points.

If the ball isn't in Irving's hands late in games, the next best option might be a teenager.

"[Tatum is] mature beyond his years," Irving said. "Especially out on that basketball court, he has the ability to get to open spots and make timely jump shots as well as make drives to the basket that are kind of uncanny for a young player."

Added second-year forward Jaylen Brown: "Especially for a rookie coming in, that's been special. I feel like him and Kyrie, they kinda get going in the fourth quarter. We need those plays at the end. They're tough shot-makers and have the ability to score like the top percentile of the league so, when they get going, it's hard to stop them."

That Tatum's late-game slam came while being chased by fellow rookie Josh Jackson will only make the moment that much sweeter for Celtics fans. Jackson drew the ire of Boston's front office when he cancelled a pre-draft workout after Celtics brass, including president of basketball operations Danny Ainge, assistant GM Mike Zarren and coach Brad Stevens, had flown cross-country to Sacramento for a glimpse of the high-lottery prospect.

Jackson called it a "miscommunication" and said he wasn't aware that the Celtics were coming to see him. Added Jackson: "I'm not going to work out if I'm not prepared."

Jackson bristled at the suggestion that Boston's then-crowded roster might have left him leery of being drafted by the Celtics.

"I'm not threatened by anybody, ever," he said. "I welcome competition."

The Celtics, who traded the No. 1 pick to Philadelphia, selected Duke's Tatum at No. 3 and could get another high lottery pick for their troubles. Tatum has responded by being the only Celtics player to start all 24 games this season. He's averaging 13.7 points over 30.5 minutes per game while shooting 49.3 percent overall and 48.6 percent beyond the 3-point arc. He's also 75th overall in ESPN's real plus-minus, a high ranking for a rookie seeing so much time. The Celtics own the best record in the NBA at 20-4 overall.

Jackson is averaging 9.3 points over just 21.5 minutes per game. His defense has been excellent at times but he's shooting 39 percent from the floor. The Suns are 8-16 and only three teams (Mavericks, Hawks, and Bulls) have more losses this season.

Tatum finished Saturday with 15 points on 6-of-8 shooting. The soft-spoken rookie shrugged off the growing buzz about his late-game production.

"Just feeling the game out, just getting opportunities," Tatum said. "We found some mismatches in the fourth quarter, and I was just trying to take advantage."

Pressed on his fourth-quarter scoring, Tatum added, "I mean I guess it just happens that way. I don't really think about it. I just try to find my opportunity and then attack."

Horford, the elder statesman on Boston's roster, is impressed by what he's seen from a player who is nearly 12 years younger.

"He's very comfortable. He's not making a bigger deal than it is," Horford said."He's just going out there and playing basketball. He has a lot of confidence. He'll continue to get better, and he's been getting better since the first game of the season. I'm looking forward to what's ahead with him."

Horford had already gushed earlier this week about how advanced Tatum is at his age. Horford is eager to see how Tatum develops early in his NBA career. But Celtics coach Brad Stevens has noted how he no longer even considers Tatum's age because the Celtics need him to be a contributor to maintain their success.

"He certainly has always been a guy that's been capable of scoring; that was something that you knew coming in," Stevens said. "Everybody you talked to, and watching him, it was pretty clear that he was always going to be able to score the ball. Doing so at that moment, you never know exactly how guys will react to that, but he has. He's got a good fearlessness, he's got a great demeanor, and he's obviously extremely skilled."