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Long before the NBA's 2017-18 season tipped off, Jaylen Brown assumed the form of afterthought following the Boston Celtics' extensive roster turnover.

The transition—or perhaps it was a demotion—was understandable. Boston gave up the rights to select consensus No. 1 pick Markelle Fultz in favor of Jayson Tatum at No. 3 and a future first. A few weeks later, Gordon Hayward migrated from the Utah Jazz. Then, after all that, the Celtics struck a trade for Kyrie Irving, giving them a third All-Star.

Brown, at best, assumed fourth-wheel status, behind Hayward, Irving and Al Horford. And that's if you believed he would get consideration over Tatum, who entered the Association as the more polished offensive option.

Realistically, Brown cruised into the season as Boston's fifth-most important player. And while his status mushroomed slightly in the aftermath of Hayward's opening-night injury, Tatum's breakout has pigeonholed him to fourth in the headlines. Indeed, it is him, not Brown, who completes the Celtics' current Big Three and eventual Fantastic Four.

Pecking-order politics don't hold weight inside Boston's locker room, though. They're bar-stool banter—points of discussion and debate designed to occupy fans and pundits.

Yet, with the Celtics still lording over the Eastern Conference, credit must be doled out where its due: to Irving. To Horford. To Tatum. To head coach Brad Stevens. And, equally as important, to Brown.

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The sophomore has developed into an unsung, verging on forgotten, hero. He is neither the primary impetus of Celtics' success nor a byproduct of it. He's another pivotal part of it—just as much as the more celebrated Tatum.

If this wasn't beyond evident already, it became clear during the Celtics' come-from-behind victory over the Philadelphia 76ers in London on Thursday. They came raging back after trailing by as many as 22 points, outscoring the Sixers 87-54 from the 6:30 mark of the second quarter, to nab a 114-103 win.

Brown led the resurgence, beginning with a 10-point outburst on a perfect 4-of-4 shooting in the second frame. He finished with a team-best 21 points, two assists, three steals and game-high plus-34. (He was a plus-40 during Boston's 87-54 stretch.)

Little about Brown's performance registered as out of the ordinary for anyone who's kept tabs on him. He is the team's second-leading scorer and no stranger to injecting life into otherwise listless efforts. The Celtics have fallen behind by more than 15 points on 10 occasions, through which they're an unreal 6-4.

Even so, Brown's offensive progression can be disarming to watch.

He's more than doubled his scoring average, from 6.6 points as a rookie to 14.2, while recasting himself as a knockdown three-point sniper. He's shooting over 39 percent from beyond the arc and nearly 45 percent from the weak-side corner. Where he finished in the 57th percentile of spot-up efficiency last year, he's in the 71st this season, amassing more points per possession than JJ Redick.

Brown's improvement is most convincing, though, when he's stepping outside his complementary devices.

Hayward's absence has empowered him—along with Tatum—to create his own shots. He isn't firing as many pull-up jumpers or working out of isolation as often as his rookie running mate. And a larger share of his baskets are coming off assists (70.7 this year, compared to 70.3 in 2016-17). But he's more aggressive overall attacking on the bounce.

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This isn't to be confused with a demonstrative uptick in volume. Brown's usage hasn't jumped by much, and while he's almost doubled his number of drives per game (4.1, up from 2.3), his playing time has spiked by an identical margin.

Most of Brown's improvement is instead tied to quicker decisions. He seldom hesitates on the catch anymore. He's shooting or attacking without pause, and more of his shots come inside two seconds of his receiving possession.

When he does put the ball on the floor, he's a more reliable hub. His handle can get messy in traffic, and he's not an off-the-dribble setup man, but he's shooting a higher percentage on drives (45.5) while coughing up possession at a lower clip.

Boston has even experimented with him as a post-up option. He's already finished as many back-to-the-basket sets as last season (37), and although his efficiency has plummeted in these situations, he's earning more trips to the charity stripe.

Subtle improvements? Sure. But they matter.

If Brown is not a go-to option, he's at least no longer an active liability. The Celtics' offensive output dipped by more 8.4 points per 100 possessions with him on the floor last season. Now, they're 5.6 points better when he plays.

This development pairs nicely with Brown's defensive hustle. No one on the Celtics is covering more ground at the less glamorous end. He's guarding more pick-and-roll ball-handlers than any non-point guard and leading the team in contested three-pointers.

That he's biting on too many pump-fakes and getting torched by kick-outs is only a minor concern. He did a nice job sticking to standstill shooters as a rookie and is more likely than not feeling the ramifications of an expanded defensive role. He's spending extra time doubling back to the perimeter after providing doubles on the inside; his blown closeouts basically boil down to superfluous effort—the frequency and speed at which he's trying to help and recover.

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That the Celtics trust him with more responsibility speaks to a greater value. They're more comfortable throwing him into space and rotating his assignments. It's across-the-board activity that makes life easier on those around him—specifically Irving and Tatum.

"That's where I hang my hat," Brown said of his defense, per ESPN.com's Chris Forsberg. "But I think it's a team effort, defensively. It's very hard to guard somebody one-on-one, so it's just everybody being engaged, defensively, and I just try to be one of those players."

None of this suggests Brown is a finished product. He's not. Horford and Marcus Smart remain the Celtics' two best defenders. And the offense won't lean on Brown for buckets down the stretch. That honor will belong to Irving, Tatum, Horford and Smart before him.

At the same time, Brown is capable of going it alone. The offense craters when he takes the court without Boston's Big Three, but the defense's stinginess remains intact. He'll continue to shape the Celtics' identity even if he never becomes a greater offensive force.

And who's to say he won't make more strides as a scorer? He's only 21 and already upping his profile.

For as much as Irving, Horford, Tatum and Hayward mean to the Celtics' future, Brown is right there with them—a two-way prospect who deserves to be billed as more than an afterthought.

Unless otherwise cited, stats courtesy of NBA.com or Basketball Reference and accurate leading into games on Jan. 11.

Dan Favale covers the NBA for Bleacher Report. Follow him on Twitter (@danfavale) and listen to his Hardwood Knocks podcast, co-hosted by B/R's Andrew Bailey.