BOSTON — Lonnie Walker IV always seems to be smiling. But his grin only grows wider when presented with an image he posted to social media after signing with the Boston Celtics in August.

It’s a snapshot of a young Walker, decked out in a Celtics winter cap, standing in the balcony at TD Garden. He is, of course, smiling.

Lonnie Walker posted a picture of him as a kid wearing Celtics gear pic.twitter.com/32PMefhvb0

— Ian Inangelo (@iinangelo) September 10, 2024

“This is one of my first NBA games with my godfather, Geo,” Walker told NBC Sports Boston. “My godparents are Italian so they are hardcore Boston Celtics fans, to say the least. So I kind of grew up watching the Celtics and this is actually one of my first [NBA] games I got to watch.”

Walker, a native of Reading, Pa., made the 400-mile trek up to Boston to visit TD Garden during the most recent Big Three era.

“They knew how much I loved basketball,” said Walker. “Watching Paul Pierce and all of them play, this was really my first time watching the big leagues and I was like, ‘Wow. I have so much to learn, so much to grow.’”

Looking at the No. 12 Celtics jersey on his chest, Walker’s smile widens again.

“This is a very full circle moment for me,” he said.

Eyebrows across the league went skyward in late August when Walker committed to join the defending champion Celtics on an Exhibit 10 deal, which is essentially a training-camp invite for a player hoping to make a team’s roster.

Celtics brass was clear that, with 14 players already signed and the team set to navigate harsh luxury tax penalties, there was no guarantee they would fill their 15th spot. Walker came to Boston knowing a more likely scenario would be getting waived at the end of the preseason and landing with the team’s G-League affiliate in Maine.

There are dozens of players in camps this month on Exhibit 10 deals. Few have 322 games of NBA experience and have proven themselves to be reliable bench-scoring options at this level.

Walker, the 18th pick of the San Antonio Spurs in the 2018 NBA Draft, could have sat around waiting for a team to offer him a guaranteed spot, or simply sulked when nothing emerged. Instead, he embraced the challenge of once again proving he belongs on an NBA roster.

“The road’s a little bit different for me, personally,” said Walker. “I’m on the E10 right now, still trying to grind and get it all together and get the guarantee, but there’s nothing better than being with a winning team, nothing better than having a culture that has a winning mindset, and that’s something that builds you, individually and overall for the team.”

And why is he willing to bet on himself?

“Mental fortitude,” said Walker. “That’s something that I really instill in myself. Each and every day I believe in myself, the amount of work that I put in, the amount of time and sacrifice that I put into this game. Sooner or later, the preparation has to meet the opportunity. So I’m very patient, understanding what I’ve got to do. And whenever my time is called, I know that I’m mentally and physically prepared.”

Walker averaged a career-best 12.1 points per game in 70 appearances for the Spurs during the 2021-22 season. He joined LeBron James’ Lakers the following season and made 32 starts in 56 appearances. Last year, Walker averaged 9.7 points over 17.4 minutes per game in a bench role with the Brooklyn Nets.

Lonnie is FEELIN’ IT

13 points in the first half 🔥 pic.twitter.com/7f8cenvmsX

— Brooklyn Nets (@BrooklynNets) November 11, 2023

In a league with 450 roster spots, it feels like there should be room somewhere for Walker. But he is content with the opportunity to show he can help Boston. After all, it would bring his basketball journey full circle.

But that young Walker — and maybe even the now-25-year-old Walker — never could have truly envisioned actually wearing a Celtics jersey.

“Never in a million years, to say the least,” said Walker. “It hasn’t fully hit me yet, but it for sure is a beautiful thing to see, as far as like full circle, you know? Just being at the right place at the right time and, by the grace of God, everything will go the right way.

“This is a championship organization coming off a championship year. It means a lot [to put on this jersey], and you kind of come into here with the championship mindset, not skipping no steps, being to the best of your capability, you owe it to your teammates each and every day to be 100 percent and be prepared, mentally and physically.”

The decision to join Boston was made easier by the presence of familiar faces, including Derrick White, after the two came up together in San Antonio. Walker spent part of this past offseason working out with Celtics guard Jrue Holiday and impressed the veteran with his work ethic.

“We just ended up working out together in the summer, kind of put in that grind, working. If it’s in the weight room or if it’s on the court,” said Holiday. “Just knowing the work ethic. He has the ability to do a lot of things. He’s very athletic and can score. I think being on a team like this, we can utilize his skill. I mean, who wouldn’t want to play here? …

“Very positive, very positive person. You see him. He’s always smiling. A very, very grateful dude. So really, really glad to have him here.”

Before the start of training camp, it wasn’t unusual to find Walker in the gym at 6 a.m. (“Early bird gets the worm,” he shrugs). He suggests he gets his work ethic from his father, Lonnie Walker III, who pushed him hard, including to prioritize academics as a child. It’s not uncommon to see Walker among the final players on the court after practice, often navigating long post-practice shooting drills.

Off the court, Walker loves horror films. This is his time of year. He’s subscribed to Shudder, a thriller-themed streaming series. At one point during our chat, he enthusiastically notes how we’re close to Halloween-loving Salem. But, like many non-New Englanders, he endearingly pronounces it, “Sah-lum.” Just wait until he has to try to say Gloucester. Still, getting up to Salem is on his October to-do list.

But first he needs to show Boston brass that he can help this team.

Walker may have to tweak his play style a bit if he’s ever going to land on the parent roster. His scoring potential is obvious, but his usage rate last season was a robust 23.0 percent (ranking in the 89th percentile among all wings, per Cleaning the Glass data) and he benefited from often having the ball in his hands in Brooklyn.

With Boston, he’d need to embrace being more of a catch-and-shoot guy. Walker shot 38.4 percent on all 3-point attempts last season and that number spiked to a robust 45 percent on all corner 3-pointers. His effective field goal percentage last season was 13.2 percent higher on catch-and-shoot (59.9) than pull ups (46.7).

The 6-foot-4 Walker has good size and a 6-foot-10 wingspan that should allow him to thrive as a perimeter defender. There’s another level he can tap into on that side of the ball.

Walker didn’t play much in Boston’s two preseason games in Abu Dhabi. He was among the team’s final reserves to hit the floor, which underscores his uphill roster climb. Walker missed all six of his shot attempts in 22 minutes of play. He hesitated at times and dribbled into tough shots. He needs to make quicker decisions and shoot with confidence to win over the coaching staff here.

But he likes coach Joe Mazzulla’s blunt style. On one of his first mornings at the Auerbach Center, Walker tried to engage his new coach by asking if he had watched the Canelo vs. Berlanga boxing match in mid-September.

“He was like, ‘I don’t watch pillow fights,’” Walker said with a laugh.

“Joe is one of the most, like, a quick left turn when it comes to conversations. He loves UFC but he hates boxing … That was about 8 in the morning, just to start it up, so that is the first ‘Most Joe’ thing I’ve experienced so far since I’ve been here.”

Spend any bit of time with Walker and you can’t help but feel the positivity he radiates. His path hasn’t always been easy — in 2020, he revealed the sexual abuse he endured as a child — but no bit of adversity seems to take that smile away.

Walker hopes the next step in his basketball journey is in Boston. He’s ready to embrace the detours if it’s not. All with hopes that it eventually brings this basketball journey full circle.