There are things that bother Isaiah Thomas, things that most N.B.A. players, most N.B.A. All-Stars, do not ever have to worry about.
“I’ll give you an example,” Thomas said after lacing his sneakers at a recent practice. “A guy scores on me, and it’s like, ‘Oh, he’s too small, a liability.’ A guy scores on a 6-2 guy, and it’s like, ‘That was a good shot.’ ”
It seems as if it has always been that way for Thomas, the Boston Celtics’ 5-foot-9 point guard, forever the smallest player on the court, perpetually seeking respect.
Last week, Thomas was named a reserve for the Eastern Conference All-Star team. The selection was another moment of validation, another example of how he has embraced the doubters to use as motivation.
“You do the same thing that a guy who’s 6-1 or 6-2 does, but there’s still always a question,” Thomas said. “No matter what level you’re at, it’s always ‘Well, he probably can’t do that at the next level.’ Why is it always a question?”
Being a small player can be a burden in the N.B.A., but it also enters you into a small fraternity. Out of the thousands of players to have appeared in an N.B.A. game, only 23 were 5 feet 9 or shorter, and only 11 appeared in more than 100 games.
Thomas, 26, said one motivating factor was dispelling for younger players the notion that short guards cannot contribute. And in the same way, he acknowledged a debt to the undersize men who played before him.
Muggsy Bogues, at 5-3 the shortest player in the league’s history, has become a mentor and sounding board of sorts for Thomas since the two met years ago, when Thomas was still at the University of Washington.
Bogues said they spoke on the phone often when Thomas, who was drafted by the Sacramento Kings and had a brief stint with the Phoenix Suns, was traded to the Celtics last year and was unsure how the move would work out.
“He was coming off the bench, and he was feeling some kind of way about that,” Bogues said. “I said: ‘Keep making the guys better, keep being a pest on defense, and most importantly, keep doing what you do: You’re a scorer.’ ”
Thomas has become the main engine of the ascendant Celtics. This season he has distinguished himself as one of basketball’s elite scorers. On Tuesday night against the Knicks, Thomas scored a game-high 20 points in the Celtics’ 97-89 win while adding 8 assists and 5 rebounds.
Thomas uses quickness and deception to elude defenders. He has an acrobat’s body control at the hoop.
Thomas said he borrowed his signature move — a half-turn, fake-360 hesitation dribble — from watching Dennis Chism, a player on the AND1 Mixtape Tour who was better known as Spyda, while Thomas was growing up.
“He finds gaps that other players can, manipulating his body around the rim, absorbing contact and managing to finish,” Knicks center Robin Lopez said of Thomas.
Lopez added with a smile, “I just think about how hard I have to try night in and night out to score, and I’m 7 foot, 7 foot 1.”
Thomas, the 60th and final selection in the 2011 draft, is, according to ESPN, the lowest-picked player to make an All-Star team since 1989, when the draft contracted to two rounds. Along with Calvin Murphy, a 5-9 guard who played from 1970 to 1983, Thomas is the shortest player to be picked for an All-Star team.
Murphy has met Thomas on a couple of occasions and has otherwise admired his game from afar. Murphy said it was necessary for him, as an undersize player, to have a bullish mentality, a surplus of confidence. He said he saw that in Thomas.
“I had a chip on my shoulder, a Napoleon complex, and I was coming at you,” Murphy said. “I could do anything the big boys did. I could dunk the way they dunked. I could shoot as well as anyone. My ball-handling was superb. It sounds like I’m egotistical, and I am.”
Murphy, who was inducted into the Hall of Fame in 1993, said he thought Thomas would become a superstar. “I love his game face,” he said. “I see he’s got the Calvin Murphy syndrome.”
Thomas has tried to hone that mental approach and has recently become obsessed with the martial artist Bruce Lee, who was 5-7. Lee, like Thomas, attended Washington. Thomas watched Lee’s movies and clips on YouTube and contemplates his inspirational quotes.
When Thomas returns home to the Pacific Northwest this off-season, he would like to visit Lee’s grave site in Seattle.
“He was so mentally locked in to everything he did, and he did it his own way,” Thomas said. “This summer I just tried to study the things he did and how he focused. The best players have the best mentality. You can’t break them no matter what situation they’re in.”
Thomas has accepted guidance and inspiration from all over. Growing up, he was a fan of Damon Stoudamire, a 5-10 point guard from Portland. He watched tape of Stoudamire’s highlights and marveled at how much of his game he had absorbed into his own.
Gary Payton, a Hall of Fame point guard, encouraged Thomas to work on his defensive game, telling him his low center of gravity was an advantage.
One of Thomas’s closest confidantes in recent years has been Floyd Mayweather Jr., a 38-year-old boxer who stands 5-8. The unlikely friendship came from a chance meeting.
In the summer of 2010, Thomas accepted an invitation from a mutual friend to watch Mayweather train at his gym in Las Vegas. Mayweather, a basketball fan, requested that Thomas call him later that day.
“I called him,” Thomas said, “and he was like, ‘Do you want to go to a Robin Thicke concert?’ ” I was like, ‘I mean, I guess so.’ He said, ‘I’ll be outside your hotel in five minutes.’ ”
Mayweather picked up Thomas, and the two hit it off right away. “It was nice,” Thomas said of the concert. “He was talking the whole time, so I didn’t really get to listen to Robin Thicke. But it was nice.”
Thomas said he and Mayweather texted or spoke about every other day. Mayweather attended a recent Celtics game and afterward accompanied Thomas to dinner. They stayed at the restaurant talking until 4 a.m.
“He tries to help me when it comes to my mentality, thinking like I’m the best, preparing like I’m the best,” Thomas said.
Thomas will be among the best players when he arrives at the All-Star Game this month. He exhibited a measure of the boxer’s bombast when he described the feeling of validation.
“Now I’m starting to get the respect that I feel like I deserve,” Thomas said. “And it’s about time.”
Correction: February 3, 2016
An earlier version of this article described incorrectly the change in the N.B.A. draft in 1989. The draft contracted to two rounds then, it did not expand to two rounds.