Heading into his second Celtics training camp, Jordan Walsh looked like an afterthought on a loaded, veteran-laden roster.

Walsh had played five times as many games in the G League than the NBA as a 19-year-old rookie, and he was coming off a miserable showing at the Las Vegas Summer League, during which he shot 29.1% from the floor and 13.3% from three while leading the team in turnovers. His odds of cracking head coach Joe Mazzulla’s rotation appeared slim.

That was three weeks ago. Now, as Boston prepares to begin its title defense next Tuesday against the New York Knicks, Walsh looks like a player who not only will avoid another season in Maine, but could have a real role for the reigning champs.

The 2023 second-round draft pick was the surprise standout of this Celtics preseason, logging the second-most minutes of any player (behind only Payton Pritchard) and playing with far more comfort and confidence than he’d shown previously.

Walsh was part of Boston’s second unit in three of the five exhibition games — including Tuesday’s finale in Toronto, in which only he, Luke Kornet, Xavier Tillman and Neemias Queta saw meaningful minutes off the bench — and started another.

Over his five appearances, he shot 45.5%, hit 36.7% of his 3-pointers, was 8-of-9 from the free-throw line, ranked second on the team in offensive rebounds (eight) and turned the ball over just twice, adding eight assists, four steals and three blocks.

Most of Walsh’s limited minutes last season came in blowouts, but the youngest player on the Celtics’ roster hasn’t looked out of place while sharing the floor with their regulars.

“Just having an understanding of what his job is: defend at a high level, defend multiple positions, rebound and then make reads on the offensive end,” Mazzulla said after Walsh turned in a 16-point, 10-rebound double-double Sunday night against the Raptors at TD Garden. “He’s doing a great job. Just really proud of his growth.

“Him, his player development team, the work they put in with him — he works hard at it, but starting to carve out a role and just a really good attitude towards it.”

The Celtics coach’s point about Walsh understanding his job is an important one. The 20-year-old admitted he took the wrong approach during Summer League, putting up 11 shots and six threes per game as he tried to be his team’s top scoring threat.

He’ll never be in that position with Boston’s big club, which features scorers galore. But with his athleticism and 7-foot-2 wingspan, Mazzulla believes Walsh can make a real impact as a dirty-work defender with occasional offensive upside if he buys into that role. He did that this preseason, with promising results.

“It feels good to have a year under my belt,” Walsh said. “I kind of know what to expect now with the season, with the team. I know how to play with the team. So I feel like that’s the biggest part going into this next year.”

Walsh nearly was the hero of Tuesday’s preseason closer.

After the Celtics’ veterans had been shut down for the night, he converted a tough, contested, game-tying layup with less than a minute remaining. He had a chance to win it on a similar drive as time expired but had shot one blocked by 7-footer Branden Carlson, clinching a 119-118 victory for the Raptors.

“Just really impressed with his poise,” veteran big man Al Horford told reporters postgame. “I know it’s the preseason, but we have to acknowledge — I feel like the game has slowed down a little bit for him. He continues to make all the right reads at the right times. (He’s) also shooting the ball really well, but also knowing when to shoot and when to pass.

“Little things that you would assume that every player has to do, but it’s very difficult, and he’s doing it on the spot. He’s competing on the defensive end. It’s been very encouraging to see his growth from last year to this year.”

Exactly what Walsh’s usage will look like once the regular season tips off remains to be seen. The 24 minutes he logged during Tuesday’s dress rehearsal in Toronto weren’t a valid preview since Boston sat both starting guard Jrue Holiday and, more importantly, backup wing Sam Hauser for that game. Hauser and Pritchard will be the first two non-bigs off the bench most nights, as they were last season.

But Oshae Brissett’s offseason departure did open a spot at the back end of the Celtics’ rotation, and Walsh now looks like the clear favorite to fill it. Brissett didn’t see much action during last season’s playoff run but played 630 minutes across 55 regular-season appearances. Svi Mykhailiuk, now with Utah, also logged 413 minutes as a deep reserve last season.

That pair ranked 12th and 13th among 2023-24 Celtics in minutes per game, so the roles they vacated aren’t large. And the 11 players above them all are still on the roster. They’re far more prominent than the minuscule role Walsh had last season, though, and he stands to benefit the most from their exits.

“It’s been fun to watch him grow,” Mazzulla said.

Boston’s other rotation hopefuls all are off to shakier starts.

Guard Jaden Springer played the fifth-fewest preseason minutes of any Celtics player. He was a DNP in two of the final three games (after joining Walsh on the second unit in the first two) and appeared to draw Mazzulla’s frustration at times. First-round rookie Baylor Scheierman couldn’t shake his Summer League shooting struggles, going an ugly 4-for-24 overall (16.7%) and 3-for-17 from three (17.6%). Veteran wing Lonnie Walker IV, in camp on an Exhibit 10 tryout contract, turned in two strong performances at TD Garden but didn’t see the floor Tuesday night.

Walker could provide value as an off-the-bench scorer with plenty of NBA experience, but the Celtics’ precarious luxury tax situation works against him, as adding him to the regular-season roster would force the team to eat close to $10 million in tax penalties. The G League is his most likely landing spot, at least to start the season.