WALTHAM — Jaylen Brown and one of his more celebrated mentors, the other Isiah Thomas, had a conversation on the eve of the playoffs, and the Hall of Famer’s advice was geared to a rookie whose playing time and ego had recently taken a hit.

Work on your free throw shooting. Focus on becoming a better defensive player.

“Those are the things that are going to get you on the floor in the playoffs — defense and free throws,” said Thomas, now an NBA analyst for Turner Sports.

Brown’s routine was jolted by an unusual emergency in the fourth quarter of the Celtics’ Game 1 win over Washington Sunday. Marcus Smart, frustrated with himself after fouling Bojan Bogdanovic on a four-point play with 7:08 left, asked out of the game. After initially sending in Terry Rozier, Brad Stevens changed his mind and subbed in Brown a minute later.

Brown, who had only averaged 4.5 minutes in the last five games of the Chicago series after a rather shaky 11-minute appearance in his playoff debut, found that everything can still click when you come in cold. He hit a key corner 3-pointer 43 seconds after entering the game, and before the Celtics’ rout was complete had pulled in two key defensive boards, assisted on a Jae Crowder 3-pointer, and did his part on Bogdanovic before the latter was subbed out with 4:10 left.

“At the time of the game it was important because Washington was on a run,” Brown said. “We were up big and they started coming back. Bogdanovic started to get going, and coach tapped me on the leg — I hadn’t played all game — and said go lock his ass up. He didn’t score a point after that.”

Brown has turned to icons like Thomas with the intent of developing his mental edge from a young age, including former Kings star Shareef Abdur-Rahim, who attended the same Atlanta-area high school. He has a mental skills coach named Graham Betchart, and a great hunger for learning how to focus and stay patient.

As his playing time started to dwindle with the start of the playoffs, Brown especially needed this last quality.

“Just try to be positive. I want to play — I want to play bad. I trust coach, but still, being a player and a competitor I want to be on the floor,” he said. “I put everything else to the side and focus on the positive aspect, because when I do get my chance I want to be ready for it. I don’t want negativity or doubt locked into my view. My chance is going to come sooner or later.”

Brown knows, because Thomas has told him so.

“It’s a lesson. Isiah has helped me since I’ve been in college — an advisor I can call at different stages,” he said. “He’s been a coach, a player, a GM, an owner. He’s a great resource for me. So I gave him a call before the playoffs started and he gave me some things. So far it’s working.”

Though Brown hasn’t received the exposure or playing time of rookies like Joel Embiid and Dario Saric from Philadelphia, and Milwaukee’s Malcolm Brogdon, Thomas has stressed that the quality of his time has counted for more than volume.

“He’s had a great experience — being able to be a part of it with his teammates and coaches at a time like this,” said Thomas. “Him and Brogdon, from the standpoint of the teams they’re on, have been able to make key contributions on good teams. If winning counts for anything, and in my book it does, then this is what it produces.”

It also helps that Brown’s locker is next to the best talker on the team, Gerald Green. The 30-year-old veteran, who has played everywhere from Russia, to China, the NBADL and eight NBA teams, has shared a lot with Brown.

“He’s been in my ear a lot,” said Brown. “Always a great vet, a positive person on the sidelines, not even just (about) this year — also about what I can be, how to work, be humble and just listen a little more. He talks to me every day. He’s helped me out a lot this year. A great person to talk to and listen to.”