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The Celtics have agreed to a four-year, $115 million contract extension with fourth-year forward Jaylen Brown. With the signing, Brown becomes the first player to agree to a rookie-scale extension with the Celtics since Rajon Rondo in 2009.

Brown’s agent, Jason Glushon, had been at the team’s training facility for several days working on the deal. When practice was opened to reporters Monday afternoon, Glushon was seen sitting in the office of Celtics president of basketball operations Danny Ainge.

“Jaylen has made tremendous strides over the last three years and has become a fantastic player for us on both ends of the court,” Ainge said in a statement. “He’s a great person and hard worker who, at 22 years old, can score and defend against the best players in the NBA.

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“Jaylen is a true professional who did a great job accepting his role last season, and he is a major part of our championship goals.”

According to a league source, $103 million of Brown’s deal will be fully guaranteed, with the final $12 million based on various incentives.

If the Celtics and Brown had been unable to come to terms prior to Monday’s 6 p.m. deadline, he would have become a restricted free agent at season’s end, meaning the Celtics would have had the chance to match any offer sheet he signed with another team.

Now that will not be a concern. The lucrative deal is an indication of the Celtics’ belief that Brown, who turns 23 Thursday, can turn into a true star.

Brown had operated without an agent since the Celtics selected him with the third overall pick of the 2016 draft, but salaries for first-round picks are locked in and thus require no negotiations. He recently hired Glushon, Al Horford’s longtime agent, to handle his extension talks with the Celtics.

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Last season he averaged 13 points and 4.2 rebounds in 25.9 minutes per game. He had a strong performance for Team USA at the FIBA World Cup this summer and is in line for a more prominent role this year after the departures of Horford, Kyrie Irving, and Marcus Morris.

In recent weeks, Brown had mostly declined to publicly discuss a possible extension, saying he was just focused on the upcoming season. On Thursday night, Ainge also declined to discuss Brown specifically, but gave a possible hint about the future.

“I think that when you’re doing a contract extension, you’ve got to feel good about it, and the players have to feel good about it,” Ainge said. “And sometimes it’s hard for you to feel good. Sometimes you need the market to dictate what it is you’re going to get paid.”

Earlier Monday, the Kings agreed to a four-year, $94 million contract with sharpshooting guard Buddy Hield, who seemed likely to receive a deal in the same ballpark asBrown’s.

Brown will now be under contract with the Celtics through the 2023-24 season, and next year the focus will shift toward a rookie-scale extension for Jayson Tatum, who will likely command a five-year maximum-salary deal.

Chad Finn: Celtics’ fate this season hinges on three players

Green on board

Ainge informed forward Javonte Green on Saturday that he had indeed claimed Boston’s final roster spot after the team waived Max Strus.

“It’s been a tough journey,” Green said Monday. “I started where I started from, to get here . . . it’s a dream come true. The feeling was amazing. All tears of joy since Saturday, but it still hasn’t hit me yet. When it hits me, I’ll get back with y’all.”

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Green, a high-flying defensive specialist, received just one Division 1 scholarship offer and then went undrafted out of Radford in 2015 before spending four years playing in Europe. He thrived on the Celtics’ summer league team in July and parlayed that into a partially guaranteed deal that is now fully guaranteed this year.

“This is just the beginning, just to get in the door,” Green said. “My job is to maintain my position here and just get better each and every day and bring to the team what the coaches want.”

Hayward sits out

Gordon Hayward missed Monday’s practice because of an illness. Robert Williams (concussion) took part in noncontact drills and is expected to be cleared for full contact Tuesday, making him eligible to face the 76ers Wednesday. Romeo Langford (knee) took part in noncontact portions of Monday’s practice.

Related: Predicting the 2019-20 NBA season

Dan Shaughnessy: This year’s Celtics have a feel-good vibe. Is it grounded in hope or reality?


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