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Over the past couple of years, the NBA's championship table has played home to a party of five.

The top-heavy Western Conference offers up the Golden State Warriors, Los Angeles Clippers, Oklahoma City Thunder and San Antonio Spurs, while the Eastern Conference needs only one seat for whatever team LeBron James headlines.

Soon, though, the league will need to make room for another championship hopeful. And though teams such as the Atlanta Hawks (60 wins in 2014-15) and Toronto Raptors (Eastern Conference finalists in 2015-2016) have flirted with crashing the party, the Boston Celtics are best positioned to join the Association's untouchables first after signing Al Horford on Saturday.

Superstar-Sized Leap

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Not five minutes after The Vertical's Adrian Wojnarowski declared the Hawks favorites to retain their frontcourt star, Horford announced his move, in emoji-speak, on Twitter. Yes, it happened that quickly, as Bleacher Report's Andy Bailey showed:



Horford's deal with Boston will be for four years and $113 million, per Wojnarowski. It says a lot about what the Celtics have built that they sold an All-Star on relocating as his incumbent team offered more years, per Wojnarowski.



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This move gives Boston its first real superstar—the same superstar Jae Crowder claimed this group didn't need.

"We all play together," Crowder said in February, per ESPN.com's Chris Forsberg. "It's a scary thing when a team don't know who to match up to, whose night it's going to be on the offensive end. And, defensively, we all fight together and play together. It's a scary approach."

He wasn't, and isn't, wrong. But now Boston is scarier.

Slotting Horford at the 5 gives the Celtics, who ranked fourth in points allowed per 100 possessions for 2015-16, the look and feel of a top-three defense. Those maintaining he is still best suited at power forward are wrong. Around 86 percent of his minutes have come at center since entering the NBA, and he contested more shots at the rim last season than all but five players.

Horford has three-point range, passes better than most bigs, doesn't need the ball in his hands to make an impact and polices the paint. He is equal parts superstar and complement, which makes him a frontcourt unicorn.

In 2015-16, Horford became just the fifth player to tally at least 1,000 points, 250 assists, 100 blocks and 80 made threes. He joined the ranks of Lamar Odom (2000-01), Dwyane Wade (2008-09), Kevin Durant (2012-13) and Draymond Green (2015-16).

Boston gets to add that kind of player to a core that is already built to compete.

Existing Promise

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The Celtics, like most teams, still have a catalog of questions that need answers over the offseason. Evan Turner is gone after signing with the Portland Trail Blazers, per Wojnarowski. Both Jared Sullinger and Tyler Zeller are restricted free agents with undefined market values. Jonas Jerebko and Amir Johnson have non-guaranteed deals for 2016-17 as well.

Social media tore into team president Danny Ainge for his relative inaction on draft night. Armed with three first-round picks, Boston dominated the rumor mill yet failed to broker the expected blockbuster. (Burning two of those three first-rounders on draft-and-stash candidates Guerschon Yabusele and Ante Zizic didn't help matters.)

But Ainge was not, and still isn't, subject to mandated impulsion. There is little reason for any team to forfeit assets just for the sake of making a move during an era dominated by indelible powerhouses.

Tiebreaker nuances left Boston with the No. 5 seed in the East's playoff bracket, but it finished in a four-way stalemate for the conference's third-best record with the Hawks, Charlotte Hornets and Miami Heat (48-34). It also joined the Thunder and Warriors, two universally recognized goliaths, as the only teams to rank inside the top 13 of offensive efficiency, defensive efficiency and pace.

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Up and down the Celtics roster, modestly priced players and prospects made understated contributions. Aside from Horford, none of them is a conventional superstar, not even All-Star Isaiah Thomas. But, together, they coalesce into a daunting collective, which includes up-and-coming NBA Coach of the Year candidate Brad Stevens.

Just four players finished the season matching Crowder's point (1,038), rebound (373), assist (135) and three-point (122) totals: Stephen Curry, Paul George, James Harden and Kawhi Leonard.

Thomas wrapped 2015-16 as one of five players to top 20 points and six assists per game while posting a true shooting percentage—the cumulative measurement of two-point, three-point and free-throw efficiency—better than 56. His company? Curry, Harden, James and Kyle Lowry.

With Thomas on the floor, the Celtics scored like a top-seven offense. Without him, they fared like a bottom-three dumpster fire. His importance to their livelihood ranked favorably among some of the league's top point men:

Marcus Smart, shooting struggles and all, was one of four players to collect 12 points, five rebounds, four assists and two steals per 36 minutes, joining the likes of Curry, Rajon Rondo and Russell Westbrook.

Sullinger, though he may not stay now, is now among five active players to amass 2,800 points, 1,900 rebounds, 450 assists, 170 steals and 125 made triples through the first four seasons of his career. His statistical siblings are or were all superstars: Kevin Durant, James, Kevin Love and Dirk Nowitzki.

Avery Bradley, in his quest to be the quintessential backcourt three-and-D stud, eclipsed 15 points per game, recorded a steal percentage of 2.2 and drilled more than 140 treys without letting his effective field-goal rate—combined two- and three-point efficiency—fall below 52. Curry is the lone other player who did the same.

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Short-burst extraordinaire Kelly Olynyk proved to be Boston's second-best two-way player. He and Crowder were the only Celtics to post an above-average box plus-minus—which shows how much better per 100 possessions a team is with a given player on the floor—on both offense and defense. Leonard, meanwhile, was the only player to beat or match Olynyk's steal (1.8), block (1.8) and three-point (40.5) percentages.

Leave this team alone and it already had enough to enter the East's contenders circle. The Cleveland Cavaliers seemed to remain on a different plane. Except now, after poaching Horford, Boston may change that.

Blockbuster-Ready

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Despite a relative inability to strike a blockbuster trade since hitting reset, the Celtics can cobble together an enticing offer if and when another superstar does hit the chopping block.

Boston failed in its attempt to pry Jimmy Butler from the Chicago Bulls ahead of the draft but won't stop trying, as ESPN.com's Marc Stein divulged to colleague Zach Lowe on a recent episode of The Lowe Post. And any botched superstar pursuit says more about Ainge's willingness to part with assets than it does about the assets themselves.

Teams that are unloading superstars would be lucky to get their hands on one of the Celtics' many cost-controlled contributors. Olynyk and Smart are still on their rookie scales, as are first-round prospects R.J. Hunter, Terry Rozier and James Young. Jerebko ($5 million) and Johnson ($12 million) are, for now, great pot sweeteners and salary-matchers, since their cap holds are non-guaranteed.

As a tweener forward who shot just 30.4 percent on two-point jumpers at California, according to Hoop-Math.com, Jaylen Brown was a reach for Boston at No. 3. But he remains a high-end lottery prospect who, after factoring in restricted free agency, can be under a team's control for the next seven to nine years.

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Bradley (two years, $17.1 million), Crowder (four years, $28.2 million) and Thomas (two years, $12.8 million) are each owed less than half of what they'd fetch on the open market. Next season alone, the New York Knicks will pay more for Derrick Rose ($21.3 million) than the Celtics will for all three ($21.1 million).

Most importantly, the Celtics, in addition to their own first-rounders, have ownership of the Brooklyn Nets' next two picks. They have the right to swap in 2017 and control the selection outright in 2018. That pick swap is now particularly inviting after the Nets shipped Thaddeus Young to the Indiana Pacers; Boston is likely looking at swiping another top-three talent.

There isn't a team that can realistically outbid the Celtics in a rumor-mill war if Ainge is approaching negotiations at full bore. The Denver Nuggets, in all their depth-chart glory, come close. But the Brooklyn picks push Boston over the top.

Owning the Future

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So much about the Celtics' future hinges on what-ifs, even after landing Horford.

Key free agents can price themselves out of Boston; it's almost impossible to imagine Sullinger returning with Horford on the books. We don't know what the team has in prospects such as Brown, Hunter and Rozier, and there is something to be said about Ainge's failure/ability/refusal to parlay the team's platter of spare parts into an established cornerstone.

There are also no assurances that Thomas, the Celtics' onetime best player and incumbent floor general, works as a complementary option with another player taking the ball out of his hands. As BBall Breakdown's James Holas underscored:

Still, the Celtics aren't chained to any one scenario or play. Their rise isn't contingent upon retaining Thomas. It isn't even dependent on consolidating assets.

As we outlined in our summer-spending report, the Celtics can carve out a second max-contract slot if it parts ways with enough free agents and non-guaranteed holds. Though they haven't been a hotbed for outside talent, they did coax Horford into leaving Atlanta. And they made Durant's short list of free-agent meetings before that, according to TNT's David Aldridge.

Available stars, quite simply, are taking notice of what they've built, per GateHouse Media's Scott Souza and MassLive.com's Jay King:

Maybe Boston won't make another splash this summer. The requisite contracts will be absurd even for stars with defined ceilings, and Ainge hasn't yet emptied his treasure trove of assets for a single trade.

But the Celtics are ahead of the curve even then. This core may not have enough to overthrow the Cavaliers, but chasing a top-two playoff berth is hardly an unwelcome ceiling. The worst-case scenario has Boston treading water with Horford, only to remain in a position of power for 2017.

Next summer's cap bump will pale in comparison to this year's, per Lowe. Most teams that spend now won't have the coin for a more star-studded free-agency class. The Celtics, thanks in large part to their platoon of below-market contracts, will have no issue manufacturing max room again while accounting for Horford.

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In the meantime, they get to soldier on as a fringe contender. And contrary to many other franchises operating in that gray area between rebuilding and competing, they aren't victims to it. They're crushing it.

No door to contention is closed for these Celtics.

They're standing on the threshold of all them, legitimate NBA superstar in tow, waiting for that now-inevitable moment when one opens a little wider.

Stats courtesy of Basketball-Reference.com and NBA.com, unless otherwise cited. Salary information via Basketball Insiders.

Dan Favale covers the NBA for Bleacher Report. Follow him on Twitter, @danfavale.