Danny Ainge is revered in Boston. He’s not on Bill Belichick’s level – but he’s next in line. The Celtics president engineered the trades that delivered banner No. 17 and has presided over one of the most impressive rebuilding efforts anywhere, ever.

And in one move – the agreed-upon deal that will send the No. 1 pick to Philadelphia in a package for two high first-rounders – he has put all that goodwill on the table.

Look: There’s no such thing as a sure thing. Kwame Brown was a stud, Andrea Bargnani was the next Dirk Nowitzki, Anthony Bennett was an imposing scorer. Projecting the future of 20-year-olds (or younger) is an inexact science. And Markelle Fultz – a late bloomer who didn’t play high school varsity until his junior season and didn’t become the guy until he got to Washington – may be more unpredictable than others.

And yet – few think so. NBA executives seem sold on Fultz. “Flat-out stud,” one exec texted late Friday, when news first broke of Boston’s interest in trading the pick. “Transformative player,” texted another. The Vertical Front-Office Insider Bobby Marks – a longtime executive with the Nets – tweeted there was a “clear separation” between the first and third picks. DraftExpress’ Jonathan Givony tweeted that there is a “big downgrade” going from first to third.

Boston made the deal … for what? A cleaner shot at Josh Jackson or Jayson Tatum, two players who play the same position as last year’s first-rounder, Jaylen Brown, who was selected No. 3 overall? A chance to hoodwink Jackson-starved Sacramento into flipping the fifth and 10th pick for No. 3?

Is a blockbuster, player-heavy deal in the offing? As of now, league sources told The Vertical that isn’t the case. Boston – everyone, really – would love to get its hands on Anthony Davis. But Davis is tied to a manageable contract through 2021, and the Pelicans appear committed to giving the Davis-DeMarcus Cousins pairing a full season together.

Jimmy Butler? Boston’s already overflowing chest of assets should have been enough for the Celtics to make a competitive offer. Paul George? With no guarantee of re-signing, any offer, Boston’s included, will be quarters on the dollar.

So what gives? Theories abound. Clearly, Ainge wasn’t as impressed with Fultz as others. But why? The “they have too many guards anyway” argument is foolish. If a franchise guard is available, you don’t punt because you like Terry Rozier’s upside. One GM hypothesized that Boston didn’t think a Fultz-Isaiah Thomas pairing would work. Fine. But if Fultz develops into a 10-time All-Star, fretting over backcourt chemistry will seem silly.

Painful, too. Fultz won’t be playing in the Western Conference. He’ll be in Philly, in Boston’s division, right in the I-95 corridor. Celtics fans will get an eyeful of Fultz three or four times a year. Eventually, Fultz and Co. will be a playoff rival.

Have you seen the Sixers’ roster? Joel Embiid is one healthy season from being a regular MVP candidate. Dario Saric joined him on the list of finalists for Rookie of the Year. Ben Simmons is perhaps the most hyped prospect since LeBron James. The Sixers have been steadfast with plans to make the almost 7-foot Simmons a point guard next season. They need shooting at other positions. Fultz, a 41.3 percent 3-point shooter last season at Washington, fits the bill.

“I play with great players all the time, so whatever [Simmons] needs me to do,” Fultz told reporters in Philadelphia on Saturday. “If I’m running the floor or I’m setting the screen for him or giving it back, it doesn’t matter. I’ll be a great teammate. If that’s me on the bench clapping for him, it doesn’t matter to me.”

If healthy, Philadelphia has a playoff team next season. In three years, it’s a conference title contender.

Will it be led by Fultz? That question hangs over Ainge like an anvil. Trading top overall picks is rare. Cleveland did it in 2014, moving Andrew Wiggins with LeBron James returning and the franchise in win-now mode. The Magic flipped Chris Webber in ’93 for the third pick, Penny Hardaway. That worked out – Orlando was in the Finals two years later. In ’86, the Sixers handed the rights to draft Brad Daugherty to Cleveland for Roy Hinson and a bag of cash. That didn’t – Hinson lasted a season and a half in Philadelphia.

Legacies are defined by deals like this. Ainge’s will be, too. His executive career is now intertwined with Fultz’s playing one, forever linked. The gambling executive just placed his biggest bet, against a player few around the league would bet against.

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