Isaiah Thomas' crew jumped out to a 18-point first quarter lead over the Chicago Bulls in Game 3 and closed out strong to secure a 104-87 win, narrowing their first-round series deficit to 2-1.
How did they do it? A major injury to the Bulls certainly helped, but Boston made one key lineup decision that unlocked the Celtics' current potential.
The Celtics went small on Friday ... and by doing so, changed the course of the 2017 NBA Finals.
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First, we have to note the absence of Rajon Rondo.
The Bulls were a completely different team without their championship-winning point guard Rajon Rondo. The offense stalled on the perimeter, as Jimmy Butler had to bail out the Chicago offense with contested prayers at the end of the shot clock.
And without Rondo disrupting the Boston offense at the point of attack, the Celtics looked more like the Golden State Warriors than the team that fell to a 0-2 deficit on its home court.
Yet as important as Rondo is to Chicago, Boston deserves a ton of credit for a seemingly minor adjustment ripped straight from Steve Kerr's playbook.
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Coach Brad Stevens turned heads Friday afternoon when he announced his starting lineup.
The conventional wisdom was that the Celtics needed to get bigger on the front line to shore up their rebounding problems. Instead, Stevens went small, inserting Gerald Green at small forward and bumping Jae Crowder up to starting "power forward."
The results were extraordinary. All of a sudden, Boston was flying up and down the court, chucking 3s, and switching on the fly on defense.
With Green, Crowder, Isaiah Thomas, Avery Bradley and Al Horford, the Celtics have five players you have to guard at the 3-point line, and the Bulls simply weren't prepared for that adjustment.
And if Chicago is without Rondo for the rest of the series as expected, it's hard to see how the Bulls will keep up with this new-look Boston squad.
Chances are, no team in the East will be completely ready for the small-ball Cs, other than the Cleveland Cavaliers. While the Wizards and Bucks can go small, Washington is more comfortable with a traditional lineup, and Milwaukee thrives on length and height.
If the Celtics can spread the floor and figure out a way to fix their rebounding woes, the NBA playoff script will get a major edit.
But can they really pull off both?
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Surprisingly, going small fixed the Celtics' rebounding woes — at least for one night.
Without real big men on the floor (sorry Al Horford, but you don't count), all five Boston players knew they had to make a concerted effort to crash the defensive glass. That gang-rebounding approach paid massive dividends in Game 3, and it could be a crucial adjustment for the rest of the Celtics' postseason run.
Simply put, Boston's front court isn't used to going out of its way to box out. These are largely finesse players who hang around the perimeter. But when they consciously decide to do the little things, the Celtics go from an atrocious rebounding team to an average one.
Given how solid they are in the other facets of the game, that little bit of improvement could turn Boston from a first-round also-ran to a conference finals contender once again.
Still, the Celtics weren't perfect. They let the Bulls back into the game late in the second quarter, then almost did it again early in the fourth quarter.
For that, coach Brad Stevens has to take some heat. He's making the biggest mistake young coaches make in the postseason by trying to play a 10-deep rotation.
In the playoffs, though, your team lives and dies with your top eight players. When the Celtics played their best guys, they built double-digit leads over the Bulls and looked unstoppable. When Stevens extended the rotation, Boston looked like a completely different team.
On the bright side, Stevens seemed to learn his lesson in the second half. He tightened the rotation (mostly, outside of that aforementioned brief stint in the fourth), and the Celtics never looked back.
So if Boston can make a deep playoff run, we'll look to Game 3 as the moment everything turned around. And if not, things still became a lot more interesting after Friday night.