Boston Celtics’ Jayson Tatum goes to the basket against Sacramento Kings’ Richaun Holmes on Monday, Nov. 25, 2019, in Boston. (AP Photo/Winslow Townson)

The Boston Celtics don’t need to face Kyrie Irving to be certain that they are over him.

Irving, who left the Celtics for the Brooklyn Nets as a free agent this summer, will be sidelined with a shoulder injury for his new team’s first visit to TD Garden on Wednesday.

While his absence spoils the night’s revenge narrative and denies the Boston crowd a prime booing opportunity, it also reinforces the central theme of the Celtics’ season: They have not been defined by the six-time all-star’s departure.

Indeed, everything about the Celtics — from their record, to their offensive balance, to their response to early adversity — suggests that they have moved on from Irving’s perplexing two-year tenure with remarkable ease.

Last year’s Celtics spent the regular season in a maddening, unresolved state before crumbling in the playoffs. Irving, who has admitted to isolating himself and feeling depressed following his grandfather’s death, performed erratically, but he was only one of many contributing factors. Gordon Hayward, an all-star caliber forward before a serious leg injury, was a shell of himself as he worked through his recovery. Meanwhile, Jaylen Brown and Jayson Tatum, Boston’s talented young wing duo, often looked ordinary. While Irving’s ball-dominant presence complicated their offensive roles, neither found much success going to the basket or playmaking for their teammates.

By contrast, this year’s Celtics look re-energized and comfortable in their own skin, boasting a 12-4 record and the NBA’s seventh-best point differential, 11th-best offense and fifth-best defense. They have compiled that portfolio despite the summer departures of Irving and all-star center Al Horford, along with another early season injury to Hayward.

To pin that turnaround entirely on Irving would be unfair, although the vacuum created by his departure has enabled many key developments.

First, it set up Boston to ink Kemba Walker as a less spectacular, but steadier, replacement at point guard. The three-time all-star has averaged 21.1 points and 4.7 assists per game, establishing himself as a lead scoring option in his preferred pick-and-roll scenarios without sucking up all the oxygen in the room.

“The Kemba pickup was huge,” said Doc Rivers, the coach who led Boston to its most recent NBA title and now helms the Los Angeles Clippers. “He’s not only a great player, but a great glue guy as well. I’m not sure anybody in the league doesn’t like him. It was exactly what they needed.”

Denver Nuggets forward Will Barton left, defends as Brooklyn Nets guard Kyrie Irving drives to the rim Thursday, Nov. 14, 2019, in Denver. The Nuggets won 101-93. (AP Photo/David Zalubowski)

The Walker-for-Irving exchange has allowed Tatum and Brown to feel greater ownership in Boston’s future and assert themselves on offense with greater regularity. Irving’s departure also removed the burden of championship expectations that seemed to swallow last year’s Celtics.

But there have been other settling forces. Brown, 23, inked a four-year, $115 million extension before the season started, and he has responded to Boston’s show of faith with career-highs in scoring, rebounds, assists and player efficiency rating.

Best of all, his offensive game is making strides toward the modern wing ideal: He’s enjoying the most prolific three-point shooting season of his career, he’s getting to the foul line more often than ever and he’s driving to the hoop nearly twice as often as last season.

“I wouldn’t have had the stomach to give him that money (before the season),” one rival scout said. “He’s making his bosses look good.”

The 21-year-old Tatum, who still has the highest upside of anyone on Boston’s roster, has shown similar progress when it comes to filling out his offensive game and updating his shot profile.

His scoring, rebounding, assists and player efficiency rating are all up this season, but this isn’t just a matter of volume. He’s attacking the basket far more often, he’s cut way back on his long 2-pointers and he’s attempting nearly twice as many 3-pointers as he did last season. As his usage rate has skyrocketed, Tatum has shown some progress as a facilitator.

“Jayson is a guy we’re going to do a lot with, and he’s continued to grow his game,” Celtics coach Brad Stevens said. “I think he can grow even more. His ability to make different reads as he continues to be put in different situations will be very important for our team, especially as he gets more and more attention.”

Hayward, whose inconsistency influenced the Celtics’ lineups and chemistry last year, started the season moving well and shooting the lights out. Boston has soldiered on with a 5-3 record since he broke his hand on Nov. 9, a promising sign given last year’s tenuous locker room dynamic.

Sometimes, a known injury can be easier for a team and coaching staff to manage than the paralyzing uncertainty caused by a recovering player. Marcus Smart has stepped in as a starter, finding clean scoring opportunities for his teammates, handling a wide variety of defensive responsibilities and keeping the Celtics engaged despite Hayward’s setback.

With a healthy Hayward, Boston has good reason to believe it can compete with the Philadelphia 76ers and Toronto Raptors as the East’s second-best team. After a humbling five-game loss in last year’s playoffs, though, it’s hard to envision the Celtics mounting a true challenge to the Milwaukee Bucks unless they pursue reinforcements at the trade deadline.

Boston’s ability to thrive defensively without Horford and Aron Baynes has been one of the league’s more surprising developments. Stevens has responded to the front line holes by turning to unconventional small lineups that turn up the pressure on opponents. This approach, born out of necessity, has been remarkably successful against a favorable opening schedule.

“We wanted to emphasize speed because we wanted to keep our five best players on the court as much as possible,” Stevens explained. “In a traditional NBA sense, most of those guys would be considered (perimeter players). We often have four (perimeter players) on the court. It’s important we use our speed. If we let the other team get comfortable, dribble around, get us lower and lower, we’re not winning. We’re just not physically strong enough to do that and we get in foul trouble.”

Boston Celtics’ Gordon Hayward (20) gets a pass off against Charlotte Hornets’ P.J. Washington (25) in Charlotte, N.C., on Thursday, Nov. 7, 2019. The Celtics won 108-87. (AP Photo/Bob Leverone)

There are obvious costs to the extreme small ball. Boston is a bottom-10 team when it comes to rebounding, and it lacks ideal individual matchups for long, physical frontcourt stars such as Giannis Antetokounmpo, Joel Embiid and Pascal Siakam. Trying to go small against Antetokounmpo will be untenable in a playoff series, and it will be problematic against Philadelphia and Toronto too.

As such, the Celtics should be viewed as one of this year’s top buyers at the trade deadline. They have movable contracts and a surplus of first- and second-round picks in the 2020 draft to facilitate a deal for frontcourt reinforcements. Boston’s front office should have more than a month before the deadline to evaluate Hayward once he returns from his hand injury, and the temptation to tool up for a postseason run will be strong if he looks as good in January as he did in October.

Boston’s realistic best-case scenario — riding a targeted acquisition or two all the way to a surprise trip to the conference finals — is compelling evidence that the organization has survived the ill-fated Irving era without lasting scars.

Many franchises would be shaken to the core if a mercurial top-15 player stormed into town, exerted total control, failed to deliver and then took off in free agency. Yet Irving came and went in Boston leaving only disappointment, not ruin.