LOS ANGELES — Those who don’t know Isaiah Thomas doubt him. Those who do know Isaiah Thomas swear by him, and are hardly surprised at his accomplishments, which have increased considerably over the past decade. Even though he hasn’t grown an inch.
Thomas is 5 feet, 9 inches and one of the NBA’s purest scorers. The newest Celtic may make his debut on Sunday night against the Los Angeles Lakers at the Staples Center.
Acquired Thursday from Phoenix, Thomas is the scorer off the bench the Celtics have craved for years and the pick-and-roll specialist who should make their offense sparkle. Twelve years ago, however, the 26-year-old Thomas was nothing more than a pint-sized player who used to hang out at Curtis High School practices in Tacoma, Wash., waiting for his varsity chance while others wondered whether he was somebody’s little brother.
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“He used to hang out, used to come out to Saturday practice as a seventh-grader,” said former Curtis coach Lindsay Bemis, Thomas’s high school coach. “He’d watch and then he’d take a ball and go shoot and I’d watch him shoot and he shot better than any of the guys that were playing. He’s not one of those kids who grew and then stopped growing. He was always small.
“He’s mentally tougher than any kids I’ve ever been around. You can’t deny him something he wants. He was one of those kids who could always rise to the occasion. He has such a strong will.
“I’ve got a 7-year-old grandson that I wish had a little more dog in him, you know? Some kids have it and some don’t. This kid, from early on, had it.”
Back then, Thomas was just the fire-hydrant shooting guard with a familiar basketball name. He emerged onto the Washington basketball scene with his performances for Curtis in the state tournament. He scored 51 points against eventual state champion Franklin and broke eight scoring records during that tournament run, including points in a tournament (241) and average points per game (40.5).
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“He just kind of had it, and he had it an early age,” Bemis said. “And he’s not afraid to fail and that’s a big thing for kids. He always did it at a younger age with older guys against bigger guys, and he was not afraid to take a shot and he was not afraid to stand his ground.”
His junior season would be Thomas’s last at Curtis. After some academic troubles, he decided to attend South Kent Prep in Connecticut. Bemis said he followed games on the Internet and saw that Thomas was up to his same scoring escapades against elite prep school competition.
Before he left Tacoma, Thomas made a promise to Bemis to come back and attend the University of Washington, a school accustomed to size-challenged, freakish athletes. Ex-Celtic Nate Robinson played high school ball in Seattle and then three years with the Huskies before becoming a first-round draft pick of the Knicks in 2005.
“When you go through a coaching journey for as many years as I have, you see many, many special players and then there’s the category of elite special ones,” said St. Mary’s assistant coach Jim Shaw, an assistant at Washington for nine years. “Isaiah is in that really elite category. I’ve coached close to 20 pros and he is unique in so many ways. He has as big a heart as anyone who wears a basketball uniform.
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“People in Boston, I’ll be very surprised if they don’t fall in love with him in a short amount of time. He has a natural charisma about him. He’s a really unique player and person. He’s going to show up ready. He’s going to show up with a great attitude.”
One moment that stands out for Bemis was during Thomas’s rookie season, when he scored 24 points for Sacramento in a loss to LeBron James and the Heat. Thomas scored 20 points in the third quarter.
On a drive to the basket, Thomas glided past James, only to become another one of his blocked-shot victims. A few days later, Bemis asked Thomas, not about being stuffed, but about his increased quickness in getting past James.
“He can score in his sleep,” said Bemis. “I didn’t think he’d be able to go into the paint and score like he does [in the NBA]. I always thought the one thing he’s good at that nobody mentions is the way he changes speeds. He worked at those things forever. He’s been doing that since he was a little kid and it’s just translated to this level. It’s amazing to watch. He certainly belongs. It’s not like he’s been given a spot in the NBA because he’s little. He can play.
“He and I didn’t finish as well [as we’d like] because he didn’t play his senior year [at Curtis]. But he’s always been respectful to me. Everything that’s happened has been good for him. I think being a Boston Celtic will be great for him. Now I’ve got to go to Boston. My wife says we’re going to Boston and I’ve never been.”
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Gary Washburn can be reached at gwashburn@globe.com. Follow him on Twitter @GwashburnGlobe.