Grant Williams opens up about his Celtics departure, his relationship with Joe Mazzulla, and more

Grant Williams (right) has always been close with Jayson Tatum, but their friendship will be a little more long-distance now.Jim Davis/Globe Staff

LAS VEGAS — Last Friday night, two days after Grant Williams learned he was headed to the Mavericks in a sign-and-trade, the forward went to dinner at a posh Las Vegas restaurant with his close friend and former Celtics teammate, Jayson Tatum.

Over the last four seasons the two developed a brotherly relationship built on sarcasm and playful ribbing, and they saw no reason to stop on this night, even though they were now rivals.

“I’m gonna bust your [butt] when we play Dallas,” Tatum told Williams.

“You think?” Williams said. “You can’t go left. And I’m gonna be physical with you, and you’ll go cry to the referees.”

It made Williams sad when he realized these moments will become much less common. The 22nd overall pick of the 2019 draft spent four seasons in Boston building relationships with everyone from Tatum to members of the support staff. Now, the people he used to see so often will be nearly 2,000 miles away.

“It was very tough leaving the players,” Williams said in an interview with the Globe. “That was probably the hardest thing, just because it’s the group that you built and accomplished so many good things with. It’s tough. And organizationally, I had a lot of great friends there. It’s definitely difficult, especially when you spend so much time in your life in a place. It’s like when you leave college, you always think, ‘Will I be able to go back and see those faces again?’”

But Williams understood it was time for a fresh start. Last fall, he and the Celtics were unable to agree on a contract extension, with Williams’s camp seeking a four-year, $54 million deal. Throughout the season, Celtics brass insisted publicly and privately that Williams, a restricted free agent, would remain part of the team’s long-term future.

That possibility appeared to be muddied the night before last month’s draft, when Boston acquired center Kristaps Porzingis from the Wizards in a three-team deal that sent guard Marcus Smart to the Grizzlies. It seemed unlikely that Boston would pay a hefty salary to bring back another frontcourt player, especially with the significant luxury-tax implications.

But Williams said his uneven role last year had already made him a bit leery of a Boston return.

“I kind of had perspective,” he said. “Even after the season and before the Kristaps trade, I had some understanding of what I was trying to accomplish in free agency, and where I saw myself moving forward.”

He had just completed a postseason in which he had fallen out of coach Joe Mazzulla’s rotation for long stretches despite making 39.5 percent of his 3-pointers during the regular season and remaining one of the team’s most physical, reliable defenders. The frustration drained him and he became more disheartened when the team struggled.

Near season’s end, veteran forward Blake Griffin, who has gone from superstar to end-of-bench role player, pulled Williams aside and commended him for learning how to be a pro when things did not go his way. Williams acknowledges that he did not handle himself perfectly, but he gained valuable lessons.

“I had a two-week span of being unprofessional, which I look back on and I’m disappointed in myself with,” Williams said. “I had to be able to realize that being professional isn’t just about when you’re having success. It’s also about your darkest days.”

For Williams, being benched by Mazzulla, who had become a close friend, could have made the predicament even tougher. Last summer, before Mazzulla had replaced Ime Udoka as coach, Williams invited him and his wife, Camai, to join him on a two-week European vacation. They biked through Madrid, Barcelona, and London, completed basketball workouts, and just enjoyed each other’s company.

The Mazzulla family keeps a prayer board in its home, and at the start of this season there was even a hopeful note pinned on it centered on Williams and his contract situation.

Then the playoffs arrived, and there were reasons for the connection between the two to become chilly. But Williams insists their friendship did not fray.

“The thing is, professionally you can always have different opinions, different outlooks about yourself and the team,” he said. “But personally, that bond never changes. I’m going to be there for him and his family and his sons any day that they need me. Personally, I always will love him as a friend. Although it may not have gone how we both expected this season, we still have a ton of respect for one another. So I’ll always speak highly of him because he deserves it. He’s a good man and a good coach.”

Williams ultimately got the four-year, $54 million deal he was seeking. It just came from the Mavericks rather than the Celtics. Now, he is expected to play a significant role alongside stars Luka Doncic and Kyrie Irving.

He sat courtside beside Mavericks owner Mark Cuban at a summer league game here last week, and his dinner with Tatum last Friday was followed soon after by another with some of his new teammates.

And make no mistake, when the Mavericks face the Celtics next season Williams will hope to remind Boston what it let get away. But he wants to maintain these meaningful connections, too.

Over the past few seasons, Williams and Tatum’s pregame warm-ups together became routine. Williams would usually smother Tatum near the 3-point line, where he’d bump and bop him to disrupt his shots while talking trash. Tatum’s 5-year-old son, Deuce, would often be there to take his dad’s side.

“I’m still gonna swat the [expletive] out of his shots in warm-ups when I come to Boston,” Williams said, smiling. “I know what time he shoots, too, so if he shoots with 90 [minutes left on the countdown clock], oh, yeah, it’s over with.”