SAN FRANCISCO — After his junior season at the University of Colorado Colorado Springs, Derrick White knew he needed to have a difficult conversation with Alex Welsh, his best friend and teammate. And Welsh, for his part, knew the conversation was coming, not that that made it any easier.
It was the spring of 2015, and Welsh had become acutely aware that White was too good of a player for the Rocky Mountain Athletic Conference — at least one opposing coach had begun calling him “the R.M.A.C. LeBron” — and for Division II basketball. It was time for White to transfer in order to play against tougher competition.
“I remember when he told me, and it was like he was super nervous,” Welsh said. “And he asked me, ‘Are you mad at me?’ And I was like: ‘No, I’m not mad. But I’m sad!’ ”
The N.B.A. finals are cluttered with former lottery picks who long ago seemed bound for greatness. Jayson Tatum and Jaylen Brown, two of White’s teammates on the Boston Celtics, were can’t-miss stars coming out of high school. Golden State teammates Klay Thompson and Stephen Curry grew up watching their fathers play in the N.B.A.
And there is White, a guard who was overlooked by Division I coaches coming out of high school and was offered only a preferred walk-on spot at U.C.C.S. because the team had run out of scholarships. His college debut was a 27-point loss in front of 211 fans in Winona, Minn., roughly speaking, about a billion miles from the N.B.A.
But late bloomers can thrive, even on the glitziest of stages, and with Jay-Z and Barry Bonds sitting courtside, White made an immediate impact in the finals, scoring 21 points off the bench for the Celtics in their Game 1 victory over the Warriors on Thursday night.
“He fits in so well with the rest of their personnel,” said Jeff Culver, the coach at U.C.C.S. “He scores when he needs to, and he plays just as well off the ball as he does with the ball.”
Ahead of Game 2 on Sunday night, White’s friends from U.C.C.S. reflected on the old days, as White emerged from obscurity to become one of the most decorated Division II players in the country. There were early flashes of brilliance, said Alex Koehler, one of his former teammates.
“We always knew he had a shot,” Koehler said. “But I didn’t know he would be this type of player.”
Before arriving on campus, White joined several of his future teammates for a game in a pro-am league outside Denver. He showed up with chipmunk cheeks because his wisdom teeth had been removed that morning. He insisted on playing, Welsh said, and wound up scoring about 25 points. One of the referees made a point of getting in touch with Culver to pass along his scouting report.
“Hey, your new guy has been the best player in the gym,” the referee told him.
Culver needed clarification: Which new guy?
“I didn’t know who he was talking about,” he said.
Culver had every intention of redshirting White as a freshman so that he could add some bulk to his lanky frame. (He had had a late growth spurt.) Culver even emailed White’s father, Richard, to make sure they were on the same page. Without White, U.C.C.S. played in a preseason exhibition game against Northern Colorado, a Division I program, and got “spanked,” Welsh said. At the same time, it was becoming clear at practice that White was one of the team’s best players. Culver had a quick chat with Jeff Sweet, one of his assistants.
“We can’t redshirt this kid,” Culver recalled telling him.
By the time U.C.C.S. made the trip to Minnesota for its season opener against Bemidji State University, White was in the starting lineup. A crowd of dozens turned out to watch the Mountain Lions get drubbed. It was not the most auspicious start to a season. White shot 5 of 12 from the field and scored 14 points.
U.C.C.S. went on to finish with a 5-21 record, losing seven games by three points or fewer. Many of the team’s narrow losses had a similar feel. White and Welsh would lead the way for 38 minutes, Culver said, then muck up the final two minutes by doing “dumb freshman things.” But their potential was enticing.
“We just couldn’t finish,” Welsh said. “We were so young, and we didn’t have any experience, and we would crumble in crunchtime.”
Yet, the season set the foundation for White’s rise. As a sophomore, he led the team to a 21-9 record and became the program’s first all-American selection. He christened his junior season by dunking over a newly arrived transfer at the team’s first practice, prompting Culver to blow his whistle — “That’s a wrap!” he yelled — before White could inflict any more psychological trauma on his teammates.
“It was like a preview of what was to come,” Welsh said.
Koehler recalled trying to defend White at practice that season.
“It was a nightmare,” Koehler said. “He could do whatever he wanted, whenever he wanted.”
During games, Welsh said, he would set “100 ball screens” for White, who would dissect his defenders (plural) in a variety of ways, depending on how they were playing him. He could launch a 3-pointer, or drive to the basket, or find Welsh rolling for an open shot. So many open shots.
“He made my life really easy,” Welsh said.
As the Mountain Lions sailed to a 27-6 record, White’s mother, Colleen, supplied the team with freshly baked cookies for its road trips. In the first round of the N.C.A.A. Division II tournament that year, White collected 50 points, 14 rebounds and 8 assists in a win over the Colorado School of Mines.
Having outgrown Division II basketball, White left to play his final season of his college eligibility at the University of Colorado Boulder. Welsh felt his absence on the court.
“He would draw so much defensive attention,” Welsh said. “I remember texting him after a preseason tournament: ‘Dude, this is so much harder without you.’ ”
Transfer rules meant White had to sit out for a season, and he spent months working to add weight. He set alarms on his phone so that he remembered to eat meals at odd hours. White was in attendance when Welsh broke U.C.C.S.’s record for career scoring. After the game, they posed together for a photograph, as White held a sign that Welsh’s family had made for the occasion: “NO. 25 IS MY HERO.”
After a standout season at Colorado, White joined the San Antonio Spurs as the 29th pick in the 2017 draft, and soon developed into a rotation player. The Celtics traded for him in February.
“Derrick is such a smart basketball player,” Tatum said. “He could fit in anywhere.”
White is part of a group chat with eight of his former teammates from U.C.C.S. They sent each other enthusiastic texts during Thursday’s series opener. There was lots of talk about “beautiful basketball,” Koehler said. White was busy during the game, so he responded afterward by thanking his friends — “He usually says something like, ‘Y’all are crazy,’ ” Koehler said — and sending three fist-bump emojis.
On Friday morning, he chatted via FaceTime for an hour with Welsh, who asked him about appearing on NBA TV’s postgame show with Shaquille O’Neal, Steve Smith and Grant Hill.
“He was mad they put him in a short chair,” Welsh said.
Culver said he was hoping to be in Boston for Game 3, while Welsh and his wife, Brooke, are planning to be in Boston for Game 4. Welsh’s dream of a Celtics sweep was still alive after the series opener.
“He wants them to win so he can be in the parade,” Culver said.
For White, it would be the latest step in an improbable journey.