BOSTON – Danny Ainge likely envisioned this moment, standing on a corner of the fabled parquet floor, congratulating a team powering into its second straight conference finals. The Celtics general manager built this group to win, spending $128 million on Gordon Hayward, dipping deep into his well of assets to acquire Kyrie Irving and using a top pick on Jayson Tatum. And so there was Ainge on Wednesday, waiting, his team streaming off the floor, a series-clinching 114-112 Game 5 win over the Sixers in the books to advance to the Eastern Conference finals.

Only instead of Irving, Ainge was glad-handing Terry Rozier; instead of Hayward, it was Marcus Smart.

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In Boston, the 1967 Red Sox season is known as the Impossible Dream. The 2018 Celtics? They are the Improbable Team.

It’s been seven months since Hayward’s season ended and a little over a month since Irving joined him on the shelf. Either injury could have derailed Boston’s chances of getting back to the conference finals; both injuries should have. “Next man up” is a cute coach’s cliché, but talent ultimately wins out.

Celtics guard Terry Rozier drives to the basket against the 76ers on Wednesday night. (AP) More

The Celtics have talent, but who knew they were this good? Game 5 wasn’t easy, and there were moments it looked like the Sixers might force the series back to Philadelphia. A punishing 87-second stretch by Boston to end the second quarter swelled a one-point lead to nine. The Sixers recovered in the third though, whittling the lead down to one. They seized a one-point lead with 3½ minutes to play in the fourth and pushed it to four a minute later.

The Celtics? They just keep coming. They came with Rozier, a backup without a single start on his resume before the season who stabilized the backcourt following Irving’s injury and has spearheaded it in the playoffs. With ex-Patriot Drew Bledsoe in attendance — and the budding bromance between Rozier and Bledsoe will keep the simmering feud between Rozier and first-round foe Eric Bledsoe on the burner for a while — Rozier was stretched to 40 minutes. In the closing seconds, with Boston up two, it was Rozier who knocked a loose ball off Joel Embiid’s leg; his two free throws on the other end extended the lead to four.

“It was tough, but that’s been our season,” Rozier said. “We overcame so much and we pulled out a lot of games like that. We knew what time it was, we never got tight, and we just pulled it off.”

They came with Smart, the soul of this Celtics team, grit embodied. Smart can be a wild ride; his perimeter shots can be ill-timed, often erratic, yet no amount of misses can dissuade him from taking them. But his defensive impact is immeasurable. He defends four positions — he stood his ground and took a charge against 6-foot-10, 223-pound Dario Saric with a minute to play — and has a free safety’s knack for finding the ball. With the Sixers looking for a game-tying bucket, it was Smart who tracked down Ben Simmons’ baseball pass to seal the win.

“He didn’t care if he ran into somebody or if he hurt himself, he was coming up with that ball,” Celtics swingman Jaylen Brown said. “I’ve said it before and [Celtics coach] Brad [Stevens] has said that if it came down to anybody coming up with it, everybody got their money on Smart.”

Added Stevens: “He’s made for this. He’s made for these moments, and that’s the thing that we just keep talking about. We can go through a stat line all you want, but when your seasons are on the line and when you’re in the playoffs and when you’ve got to do really hard things, he can do really hard things.”

They came with Tatum and Brown, the 20-something futures of the franchise who have been rushed into its present. Tatum will finish a distant third behind Simmons and Donovan Mitchell in the Rookie of the Year ballot, but he outplayed Simmons in this series and he will play longer than both. His final second-round numbers: 23.6 points on 52.6 percent shooting. Brown battled through a hamstring injury but still knocked down 50 percent of his threes.