CHICAGO — After the eighth-seeded Bulls stunned the top-seeded Celtics by winning the first two games of this opening-round playoff series in Boston, Celtics coach Brad Stevens knew he could not maintain the status quo.

So he made a seemingly risky adjustment in Game 3, inserting veteran forward Gerald Green into the starting lineup for center Amir Johnson. Green played well. He brought some energy at the start, scored 8 points, and helped Boston set the tone by spacing the floor for a perimeter onslaught.

Afterward, Stevens was praised for the surprising yet effective switch that had helped his team take a 104-87 win and forcefully proclaim that this series is far from over.

Ultimately, though, it was Stevens’s general shift in philosophy that proved most important. The Bulls held a sizable rebounding edge at the start of this series even while the Celtics played their bigger players. And when the Celtics played their bigger players, it left them weaker elsewhere, like on the perimeter.

Stevens reasoned that if they were going to give up rebounds again — and they did, getting out rebounded, 52-37 — they wanted to be in position to strike in other ways. So for key stretches he deployed a three-guard lineup consisting of Avery Bradley, Terry Rozier, and Marcus Smart, a grouping he did not use once in the first two games of this series.

And in Game 3, that unit was dominant. With those three on the floor together, the Celtics ambushed the Bulls with relentless and effective perimeter shooting and pinpoint passing.

Over 15 minutes, the Celtics made 15 of 21 shots and 7 of 12 3-pointers for an effective field goal percentage — a metric that factors in the value of 3-pointers — of 88.1 percent.

The group had a ridiculous net rating of 77.4, meaning that they were outscoring the Bulls by a rate of 77.4 points per 100 possessions. To put that figure in perspective, the Warriors led the NBA in net rating during the regular season at 12.1.

“I just thought we had good juice,’’ Stevens said. “When Isaiah [Thomas] was out we had Terry and Marcus and a lot of times Avery, all three in. So we were still pretty quick, very quick, and I thought those guys did a good job of playing together and sharing the ball.’’

With that group on the floor together the Celtics had assists on 14 of their 15 baskets.

Of course, hot shooting like that is certainly not sustainable, even for open jump shots. But for the Celtics, the flurry was probably most encouraging because it took place with Thomas watching from the bench.

During the regular season the Celtics struggled mightily to score without Thomas. When he was in the game, they had an offensive rating of 113.6, and when he was not, that figure plummeted to 99, the largest discrepancy on the team.

But in Game 3, the Celtics had a 154.9 offensive rating during the 17 minutes when Thomas was on the bench.

“To be honest, that’s something that we’ve been struggling with as a team, as a whole, the coaches, the players,’’ Bradley said. “I think it’s kind of hard when you’re in there with a group of guys that you’re not used to playing with. We’re trying to figure out. Throughout this year we’ve been trying to figure out ways to score the basketball and it’s kind of been hard for us because we don’t know how to play off each other, some of the groups that are going in there. That’s all credit to the guys that were in the game yesterday and Brad. His play-calling, I think, made it a lot easier for the players that were in the game to play off each other.’’

Bradley and Smart have had essential roles for the Celtics all season, of course, but Rozier’s chances have been more inconsistent. On Friday, the second-year guard was on the court for the final 16 minutes of the game. In 23 total minutes, he made three 3-pointers and the Celtics outscored the Bulls by 19 points.

“Terry, him being ready, it just talks about the value to our team to where everybody’s always ready and everybody’s going to contribute to help us win,’’ Smart said.

Adam Himmelsbach can be reached at adam.himmelsbach@globe.com. Follow him on Twitter @adamhimmelsbach.