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BOSTON — Terry Rozier thought he would be the Boston Celtics' starting point guard this season. Or at least that was his goal. This was in the summer, back when Isaiah Thomas was still on the roster and recovering from hip surgery.

It isn't that Rozier wanted to depose Thomas. He just wanted to prove to the Celtics they could survive, and still thrive, without Thomas on the floor.

But in August, the Celtics pried Kyrie Irving away from the Cleveland Cavaliers. Rozier was thrilled, but he also knew his dream of leading a team into the playoffs would have to be put on hold. Or so he figured.

Then came Irving's season-ending knee surgery. His absence could have been a kill shot to the Celtics' season. Many thought it was. Yet so far, the team has been able to overcome the loss. Boston squeaked by the Milwaukee Bucks in the first round of the Eastern Conference playoffs. And then, courtesy of a Rozier eruption (29 points, eight rebounds, six assists), the Celtics blew out the favored Philadelphia 76ers in Game 1 of the second round.

Rozier is averaging 19.0 points, 6.6 assists and 4.8 rebounds in eight postseason games. He's shooting 43.8 percent from deep. His efficiency numbers, from his shooting to his assist rate, have all skyrocketed even though he has the ball in his hands more frequently.

"He's got a freedom now without Kyrie," Sixers head coach Brett Brown told reporters Wednesday. "He becomes sort of what they needed in Kyrie. He's got a real ability to score. He has a dance with the ball that I think is elite. And there's a freedom that he has [that] sort of partners with his confidence under a pretty impressive skill package, and it's a perfect storm. He provides a heck of a plan B."

While he might not be the sole reason for the Celtics' surprising Irving-less run, the team likely would be watching the second round of the playoffs from home if not for his emergence.

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There were many reasons behind his career-high numbers across the board this season (11.3 points, 4.7 rebounds and 2.9 assists per game). The typical growth that comes along with increased experience, for one. Slightly more minutes, too. But the third-year guard also entered the summer with a plan, one that centered on bolstering two specific areas of his game.

Rozier connected on just 31.8 percent of his treys last season. The problem, he and his trainers recognized, was that his shot was leaving his hand flat. He was launching the ball at an angle that limited its chances of splashing through the net. To fix this, Rozier spent hours in a gym shooting with just his right hand while keeping his left hand tucked off to the side. The goal was to focus on lifting his elbow higher on his release and to generate more strength from his legs.

"For a guard like that, you have to be able to make that pull-up three," Nick Friedman, one of Rozier's trainers, said in a phone interview. "Otherwise, the defense just goes under [on screens] no matter what."

That simple adjustment paid huge dividends. Rozier drilled 38.1 percent of his three-point looks this season. More importantly, he hit 36.0 percent of his pull-up triples, a 70 percent uptick from the previous season, per NBA.com.

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That was Part I. Part II consisted of polishing his pick-and-roll game so he could take advantage of the lanes and pathways that an improved jumper would create. He worked on getting his shoulders below those of the player setting the screen so he could get downhill. He practiced aggressively turning the corner off of picks. During film study, he concentrated on watching for the proper moments to hit his roll man and how to draw defenders into the paint.

"Up early, every day, putting in a lot of work, watching a lot of film," Rozier told reporters following Game 1 when asked about his offseason routine. "I just worked hard the whole summer."

He and his trainers always believed he had the ability to serve as a lead guard in the NBA. Cody Toppert, a G League coach and former trainer of Rozier's, said that's one of the reasons Rozier participated in the five-on-five portion of his predraft combine, an event many projected first-round picks skip.

"We wanted to showcase that ability he had," Toppert said in a phone interview. "It's the stuff that only comes out in a five-on-five."

He just needed the chance to flash those skills in the NBA. Irving's absence provided it, and having spent the summer preparing to take on this role, Rozier has stepped in seamlessly.

"We always knew he had it in him," Sixers center Amir Johnson, who played with Rozier in Boston last season, told Bleacher Report. "He's a hard worker, and the opportunity came, and he's stepping up to the table. And then what happens? You get confidence as a player. Things like this happen."

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Spend enough time around the NBA, and these stories begin to blend into one another. Every player says they spent their summer spilling sweat. Every player says all they need to thrive is the opportunity.

"But I'm telling you, I've been doing this with NBA players for six years, and I've never seen a guy entering his third season approach that summer like it was his predraft time," Friedman said.

Only time will reveal whether Rozier can maintain this level of production or if it's just a hot streak. And this playoff outburst is going to present the Celtics, who suddenly have a crowded backcourt, with some difficult offseason decisions.

Rozier has one more year left on his rookie deal, but he's eligible for an extension this summer. Marcus Smart, his young backcourt teammate, is set to be a restricted free agent. Irving isn't going anywhere. Jayson Tatum and Jaylen Brown are budding stars. It'll be difficult for the Celtics to keep all of these players in town. Changes are likely coming.

But that's a conversation for a different time. Right now, Rozier is focused on one thing.

"It's been a dream come true," he said after Game 1. "I want to keep going."