Brown said he wasn't certain he'd play Thursday and even told Celtics coach Brad Stevens he didn't think he could play so soon after the death of childhood friend Trevin Steede.
The cause of death was not immediately known.
Brown said a phone call from Steede's mother convinced him to play in his friend's memory.
"My best friend passed last night. It was tough to kind of accept it," Brown said. "Everybody was kind of in shock. But I knew coming into today that he would have wanted me to play. It was hard getting my thoughts together, but after talking to his mom and his family, they inspired me to come out and play.
"I wasn't in any shape to come out. I didn't want to leave my room. But they inspired me to come out and play. And I came out and played in his spirit today. My teammates held me up and we pulled it out."
Brown posted a tweet later Thursday in honor of his friend.
Immediately after Thursday's game, Kyrie Irving sought out Brown and gave him the game ball.
"Ky gave me the game ball and said, 'This one was for Trevin,' " Brown said. "That meant a lot to me."
Said Irving: "I knew from the start of the game. I gave him some great words that was given to me. I've lost individuals in my life. It's never a good thing when someone else is going through it. You do your best to console and to encourage them, but at the end of the day, it's about the strength within themselves and he showed a lot of that tonight, to be able to go out there and perform the way he did, I knew exactly where the game ball was going to."
A clip of Brown and Steede became a bit of a viral sensation a few weeks ago as Brown dominated his friend in a game of one-on-one at the Celtics' practice facility to demonstrate the difference between a pro and casual hoopster.
Brown detailed Thursday how the two met in high school and were fast friends.
"[Steede] tells this story all the time: I transferred schools to play basketball at Wheeler High School. He was already there. And they transferred me in," Brown said. "Ultimately, I was probably taking his spot, but I remember I didn't have any friends sitting at the lunch table. The first few days I sat by myself and acted like I'm too cool, I didn't need anybody. You know what I mean? Like, I'm an introvert. I'll be all right.
"And he walked up to me on the third or fourth day and asked me who I was sitting with, even though I wasn't sitting with anybody. And then he told me to come over and sit with him. And ever since then we've been best friends. For him to do that even though the circumstances, ever since then that's been my brother, you know?"
Brown said he appreciated the support of his teammates and Stevens, who told Brown he didn't need to play if he didn't feel up to it.
Stevens said his conversations with Brown were similar to those he had with Isaiah Thomas last season after Thomas' sister died on the eve of the playoffs.
"A couple hours before [Thursday's game], I was talking to Brad, and I told Brad I didn't think I was going to be able to do it," Brown said. "I couldn't get my thoughts together, I couldn't gather myself. Then his mom called me and gave me a whole new inspiration. I kinda think that was the reason why I was able to come out and play and be controlled and poised."
Brown was expected to travel to Atlanta on Friday morning to be with family and friends. The Celtics visit the Hawks on Saturday night.
It was Brown who gave the Celtics energy as they twice rallied from 17 points down.
Said Golden State's Kevin Durant: "[Brown] was the difference tonight."
Teammates marveled at how Brown played through the tragedy.
"I'm extremely proud of him," Jayson Tatum said. "He's a tremendous player and person, and coming in today was tough, but basketball is a good distraction. He went out there and gave us exactly what we needed."
Added Al Horford: "Much respect for him, because that's difficult and he was hurting. He is hurting. It was one of those things where I didn't know if he was gonna play tonight, but it just speaks of his character. Some people deal with it different ways. Some people need to get away. He was able to put all that aside and play, so I'm very proud of him."