Not the missed dunk with 36 seconds left - thanks to Tatum rebounding it himself and making a tough jumper - and not his foolish foul with 1.9 seconds remaining.
Tatum did far more good than bad, finishing with 24 points and 14 rebounds as Boston held on to beat the New York Knicks 103-101 on Saturday night.
"I mean, that guy's just super talented," Kyrie Irving said. "To be so young, to be so poised is an awesome trait for him."
The second-year forward showed it with his calm response to his botched dunk with the Celtics clinging to a one-point lead.
"He should've made the dunk," Irving said. "I told him, 'I can't dunk it for you, I can only pass you the ball.'"
Tatum grabbed the rebound and with the clock running down made a tough turnaround jumper for a 101-98 lead with 21 seconds to go.
"I was just trying to make up for it," Tatum said. "I just missed the easy one."
He had then given the Celtics another three-point lead with two free throws with 7.9 seconds left and the Knicks called their final timeout. They had trouble getting the ball inbounded from the sideline, and Knicks guard Trey Burke had to race into the backcourt to retrieve it as the clock ran down. He pulled up for a 3-pointer from well beyond the arc and Tatum jumped to contest it, fouling Burke for three free throws that could have tied it.
But he missed the first and the Knicks couldn't grab the rebound when he intentionally missed the third.
"As a point guard, I know I can knock those down in my sleep but tonight I missed the first one. I got to live with that," Burke said. "I feel like I let my teammates down."
Irving and Marcus Morris each added 16 points for the Celtics, who rebounded from their loss in Toronto on Friday night. They won despite giving Gordon Hayward a night off to rest his surgically repaired left ankle.
Tim Hardaway Jr. scored 24 points for the Knicks, who lost rookie Kevin Knox to a sprained left ankle in the first quarter. Enes Kanter had 17 points and 15 rebounds before fouling out of their second two-point loss in two nights.
The Celtics are considered one of the Eastern Conference favorites but coach Brad Stevens said the Raptors were clearly better now and Boston needed to improve after Toronto's 113-101 victory Friday night.
On Saturday night, they were just good enough to beat the rebuilding Knicks.
The Celtics looked sharp early in racing to a 26-10 lead Saturday and still had a double-digit cushion midway through the second quarter before Damyean Dotson's second straight 3-pointer trimmed it to 50-48 by halftime.
Celtics: Aron Baynes got the start with Hayward out and scored 10 points. But he played only 12 minutes before leaving with a sore right hamstring. ... Jaylen Brown was only 2 for 9, finishing with seven points.
Knicks: Coach David Fizdale said G Courtney Lee would see specialists Monday after experiencing neck spasms that went down into his chest. Lee was sidelined throughout the preseason by a neck injury. ... Kanter became the first Knick since Zach Randolph (2007-08) to start the season with three straight double-doubles.
KNOX KNOCKED OUT
Knox was driving for a layup on a fast break when his ankle appeared to roll badly as he was fouled by Terry Rozier III. Knox stayed on the court briefly, appearing in pain, then was carried off by a pair of teammates.
Because the No. 9 pick wasn't able to shoot the free throws, he was, by rule, forced to miss the remainder of the game. The Knicks said X-rays on his ankle were negative.
Stevens said there wasn't a plan to rest Hayward on back-to-backs, but his minutes are being watched and he's being evaluated on a game-by-game basis. He said Hayward, who broke his ankle last year in the season opener, was just experiencing general soreness Saturday and didn't think it was anything overly concerning.
Celtics: Host Orlando on Monday night.
Knicks: Visit Milwaukee on Monday night.
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