With 1:27 to play in Game 5 of the Eastern Conference semifinals against the Sixers, Boston trailed, 109-105, and looked increasingly in danger of seeing a series it had a chance to close out extended by at least another game.
That was when Jayson Tatum took a pass above the left break and dribbled toward the rim, a full step ahead of his defender, Ben Simmons, who had been screened by Al Horford. As Tatum drove, Horford stepped into the other side of the lane, and when the Sixer defending Horford - Dario Saric - slid over to help Simmons, Tatum immediately looked up and floated a pass toward the rim. Horford, nearly alone, dunked the pass and ignited the crowd.
The Celtics rallied to win and move on to the conference finals against the Cavaliers. For Tatum, it was just that kind of night - he scored 25 points on 8-for-15 shooting, including the go-ahead basket with 22.5 seconds left. He was 9-for-11 from the free-throw line, and scored 10 of his points in the fourth quarter.
"It's a great feeling," Tatum said. "You work all season to earn the trust of your teammates and the coaching staff for moments like this, and you've just got to go out there and make the right play, whether it's finding Al on the lob or making the layup."
Tatum was outstanding throughout the conference semifinals, going for 20 points or more in all five games (averaging 23.6) and shooting 52.6 percent from the field even as he struggled from the 3-point line (31.6 percent).
For the Sixers, of course, Tatum's performance provided an extra bit of sting. Just about a year ago, Boston, which owned Brooklyn's pick, won the NBA's draft lottery. In the weeks that followed, team president Danny Ainge zeroed in on Tatum as his target, even as the team continued to send signals that there was interest in Markelle Fultz, Lonzo Ball and Josh Jackson.
Philadelphia had the No. 3 pick and wanted Fultz. Just ahead of the draft, Ainge was able to work a swap of picks with the Sixers, adding either the Lakers' pick this year (if it falls between No. 2 and 6) or, most likely, Sacramento's pick next year.
Tatum worked out for the Sixers in New York last spring, his first workout for a team - and conducted when the team still had the No. 3 pick. Tatum's last workout was for the Celtics, when he flew to Boston despite being sick and on antibiotics, and still impressed Ainge and the Celtics brass.
That sealed it. The Sixers took Fultz first. Boston took Tatum.
Earlier this year, Tatum told Sporting News that was no surprise. "I was aware of it through my agent," Tatum said. "The Celtics were going to take me No. 1 all along, but the Sixers didn't know that. Everything just works out for a reason."
It might be hard for the Sixers to see that reason at this point. Tatum, at just 20 years old, was certainly the best rookie in the series against Philadelphia, as presumptive Rookie of the Year Ben Simmons had some good moments, but mostly struggled, and Fultz did not get off the bench in the five games. You could even argue that Tatum, who led all scorers, was the best offensive player in the series.
"I thought he was great," Sixers coach Brett Brown said. "I think he's had an exceptional year. He's got a bounce. When I watch him, he plays sort of older than his resume suggests. He's got a lot of bounce and a lot of game. … I thought he was excellent."
For the Sixers, it had to be difficult watching Tatum torch the variety of defenders he saw - Simmons, Marco Bellinelli and primary defender Robert Covington - especially as their own offense seemed to lack a good dose of athleticism and shot creation, two areas where Tatum excels.
But the issue for the Sixers is that they're probably not done seeing Tatum in these situations. Both Boston and Philadelphia are stocked with budding young talent, and figure to be East postseason stalwarts in the coming five-to-10 years. Considering his age, Tatum is only going to improve and continually remind Philly of what might have been had the 2017 draft gone different.
There's still the chance that Fultz, who showed some promise when finally getting back on the floor after a difficult shoulder injury and a tough mental transition to the NBA, will bounce back next season and prove worthy of his potential. But there's also a chance that his jump shot will never really return and that his confidence will disappear with it.
No matter how the long-term turns out for Fultz, in the short term, the Sixers could have badly used a guy like Tatum against the Celtics. His ability to take control of the Boston offense - his usage rate is 24.8 in the playoffs, highest among the Celtics, and was 27.2 against the Sixers - speaks to how poised and polished he has become over the course of his rookie season.
He was asked on Wednesday about his journey from his debut game in Cleveland in October to now, preparing to face the Cavs with a trip to the Finals on the line.
He smiled wide at the thought. "It's like night and day," Tatum said. "I mean, the first time we played them, I was so nervous. But it's been 90 games since then, and we've been through a lot, I am a lot more relaxed and calm, but ready to go out there and play."
He was ready in the semifinals. And he sent the Sixers home, undoubtedly wondering what might have been.