Avery Bradley and Rajon Rondo were Celtics teammates for nearly 4½ seasons before Rondo was traded to the Mavericks in December 2014. They remained close even after Rondo’s departure; he was a guest at Bradley’s wedding. So when the Celtics and Bulls meet in a first-round playoff series that begins Sunday, Bradley said it will be like competing against a brother.

“It’s weird playing against him, going up against him,’’ Bradley said. “But I know he’s a true competitor and he’s going to bring his best game in the playoffs. So it’s going to be fun playing against him. Weird but fun.’’

Bradley, who is five years younger than Rondo, said he learned so much about life and basketball from his former teammate. Even though the two had diverging styles on defense — Bradley as a ball hawk and Rondo slicing into passing lanes — Rondo offered a morsel of advice at almost every practice, Bradley said.

“I feel like he was one of the best teammates that I’ve had,’’ he said. “And he was a true competitor, one of the hardest-playing teammates I played with. His desire to win, I don’t know that many people [like that]. He’s so competitive. It doesn’t matter what we’re playing. If we’re playing tic-tac-toe, he wants to win every single game. So I respect that about him.’’

Rondo became a star in Boston, helping the team to the 2008 NBA title while emerging as a key part of five consecutive playoff teams. But the stints with the Mavericks, Kings, and Bulls over the past 2½ seasons have been more uncertain and unsuccessful. Bradley said he has remained in Rondo’s corner amid the criticism.

“It was hard hearing all the negative things about him,’’ he said. “But I just prayed for him. I wished him well. I would send him texts every now and then like, ‘Bro, it doesn’t matter. We’ve all been there before.’ He’s been the underdog before. And he came into the NBA, people not thinking he could play. Everybody’s been there. It’s just their job to continue to work and prove people wrong. I feel like every year he’s been trying to do that.’’

Rondo averaged 7.8 points, 6.7 assists and 5.1 rebounds this year. Those statistics are more modest than the ones he compiled in Boston, but the Celtics are aware that Rondo sometimes saves his best moments for the biggest stages.

“You know when it counts he’s gonna be ready and he’s gonna perform,’’ point guard Isaiah Thomas said. “So we’ve got to slow him down and eliminate what he does best, and that’s getting everyone involved.’’

Nader top rookie

Celtics draft pick Abdel Nader on Friday was named the NBA D-League’s rookie of the year. Nader, the 58th pick of last June’s draft, averaged 21.3 points, 6.2 rebounds and 3.9 assists for the Maine Red Claws this season.

Celtics rookie Demetrius Jackson has spent much of this season with the Red Claws, and he said Friday that Nader’s play has been impressive.

“He can really score the ball and has a really aggressive mindset,’’ Jackson said. “He can miss one or two shots, and he’s going to keep going, keep attacking. I really admire that. And he can really shoot the ball and is one of those guys on the offensive end who can do it all. Defensively, he’s big and athletic and can make plays, and I think he’s focused on getting better at that end.’’

The Red Claws will face Raptors 905 in the D-League Eastern Conference semifinals, which begin Sunday.

Focus on Butler

The Celtics understand their success against the Bulls will hinge in large part on their ability to slow down the All-Star Jimmy Butler. Boston will likely throw waves of its top defenders at Butler at various times, including Bradley, Jae Crowder, and Marcus Smart.

“Those guys are going to give a tremendous effort, they’re going to study, they’re going to be as locked in as you can be,’’ coach Brad Stevens said. “Jimmy Butler is still going to make a shot, right? He’s still going to find a spot and raise up and make a really tough shot that you have to tip your hat and run down the court and try to score, then come back and try to do it again. You’ve just got to make it as tough as possible.’’

“Your goal is to try to make it so they have to take a lot of shots to score, or at least feel you the whole game.’’

Thomas answers critics

As dominant as Thomas has been as an offensive player this season, he has taken some criticism for the team’s poor defensive numbers when he is on the floor. Thomas, for one, isn’t bothered by the suggestion that he is a liability on that end of the court.

“That’s the people who don’t watch games, so I’m fine with it,’’ he said. “It’s the people who probably can’t play basketball.’’

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