From his baseline seat, second row, within earshot of co-owner Steve Pagliuca, Celtics president Danny Ainge watched, cringing, undoubtedly, at much of what he was seeing. All season Boston had been a resilient bunch. No lead was safe, no comeback too improbable. Yet here they were, down 3-2 in the series and watching Atlanta run roughshod on the parquet floor. The brilliance of Brad Stevens couldn’t slow it, the heroics of Isaiah Thomas couldn’t stop it, and the Celtics season ended Thursday with a 104-92 defeat.

This was a season of which to be proud in Boston. From 25 wins in 2014 to 40 last season to 48 in this one, the Celtics’ progress is evident. Ainge’s rebuilding of a decaying dynasty has been brilliant, to locking up Stevens to a six-year deal, to acquiring Thomas, to poaching Jae Crowder and a first-round pick from Dallas for Rajon Rondo a season ago. But there would be no trip to Cleveland, no shot at LeBron James, no storybook ending. Just a step forward and a sturdier foundation to build on.

Boston goes home, but make no mistake: The real work begins now. The Celtics’ reinvention has been masterful, but if the transition from rebuilding team to playoff contender is difficult, evolving into a title contender is a Rubik’s Cube of a challenge. Ainge is armed to the teeth, with $16 million in cap room – $33 million if Jonas Jerebko and Amir Johnson are cut loose – and eight picks in the upcoming draft, including three in the first round and one with a solid chance of landing inside the top three. The Celtics could continue to rebuild slowly, organically, but those who know Ainge know that just isn’t his style.

“Everybody understands and knows Danny, and the things that Danny does,” Celtics guard Marcus Smart said. “So we’re excited to see what he has in mind.”

Ainge will be aggressive in free agency, team sources told The Vertical, and yes, that means a run at Kevin Durant. The Celtics believe Durant will meet with them this summer, but they know that meeting won’t accomplish much unless there are significant moves leading into it. Durant will be 28 in September, and the former MVP isn’t interested in hearing what a team could someday become. He wants to win now, which is why Golden State and San Antonio expect to be appealing, why Oklahoma City is still very much in play. Boston needs a deal for Jimmy Butler, a commitment from Al Horford, a carrot to dangle in front of Durant to persuade him that relocating east is the smart play.

The alternative is a slower, more deliberate approach, but would that really be a lousy option? Boston was a Crowder late-season injury from 50-plus wins and the No. 3 seed, and its core should only improve. Decisions must be made on Evan Turner and Jared Sullinger, two cheap rotation players in line for big paydays. But Avery Bradley – whose absence because of a series-ending hamstring injury late in Game 1 crippled Boston’s ability to seize control of the series in Atlanta – is under contract, Thomas and Crowder, too, while Smart and Kelly Olynyk are locked into rookie-level deals.

Building through the draft is arduous, imprecise, but hey, look at the teams that do it. Golden State has a largely homegrown roster, San Antonio, too, while Portland is one win away from a trip to the second round with a couple of mid-major guards leading the way. There will be a franchise player in the top-five, perhaps one in the mid-first round – where Boston will have Dallas’ pick – as well, and the smart teams always find value in the 20s. Too many picks, not enough roster spots, blah, blah, blah, and yet who wouldn’t want to put promising talent in Stevens’ hands?

So many decisions, and these are the moments for which Ainge lives. Trade after trade has panned out, and Boston has inched closer to becoming a true threat to Cleveland as an Eastern Conference power. The next few months will put Boston on the path it will take for the next few years. The season is over, but there are so many reasons to be optimistic about the future. It’s on Ainge now. It’s his turn to deliver, again.