Celtics center Robert Williams III rips down a rebound in front of Atlanta Hawks forward Onyeka Okongwu during Game 1 of their first-round series on Saturday in Boston. Stuart Cahill/Boston Herald

On the opening montage on the Jumbotron at TD Garden prior to the start of the Celtics’ playoff opener, an ultra-serious Marcus Smart appeared on the screen, and the point guard delivered the following message: “There’s only one goal. Banner 18.”

There’s a long way to go to take care of their “unfinished business” and achieve that goal, but Game 1 against the Atlanta Hawks is in the books.

Impressions? Takeaways?

Even with a significant letdown in the second half that caused a few anxious moments, the Celtics still checked off all the necessary boxes.

Jayson Tatum (25 points) and Jaylen Brown (29 points) provided their usual 1-2 punch, as Brown shook off any discomfort caused by the five-stitch cut on his shooting hand, which actually opened up at one point and needed attention.

Derrick White, meanwhile, gave Atlanta star Trae Young (5-for-18 shooting) fits trying to produce his usual scoring output. White (24 points) even outscored Young (16 points) for good measure.

Smart also looked more like the defensive player of the year he was a year ago, with his physical presence and work on Young when White wasn’t in the game.

Ultimately, Brown, Tatum, Al Horford et al made sure the Celtics didn’t completely blow a huge halftime lead.

And with such luminaries as Robert Kraft, Dustin Pedroia, and several Bruins (David Pastrnak, Hampus Lindholm, Jake DeBrusk and Charlie McAvoy) in the house looking on, first-time head coach Joe Mazzulla didn’t have to push many buttons or outcoach Atlanta’s Quin Snyder.

Mazzulla just needed to get his team to put its foot back on the gas in the second half to secure the 112-99 win.

But peeling back from a great first half (74 points) and lackluster second half (38 points), there was at least one significant takeaway.

If the Celtics continue to get the Robert Williams that showed up in the first half, there really isn’t a ton to worry about in wake of their Jekyll and Hyde performance in Game 1.

The Celtics managed Williams’ games and minutes all season long, with hopes of him being healthy for the postseason, and he looked fresh against the Hawks.

Williams was an absolute stud, especially in the first half when the Celtics raced out to a 30-point lead before putting it in cruise control the rest of the way. This was the best Williams has looked all season, and a reminder of how important the big man is to the team’s championship hopes.

As the first player to come in off the bench, Williams was 5 for 5 with 10 points, three rebounds and two assists in the first half. He came in on fire and the Celtics fed off his energy, especially on the defensive end.

He finished with 12 points (6 for 6), eight rebounds and two assists. Along with Brown and Tatum, he triggered the first-half explosion, as the Celtics produced a 74-point barrage while imposing their will on defense.

With a 30-point lead that ultimately dwindled, Mazzulla didn’t see the need to overextend Williams, who didn’t play much in the second half.

But there’s no denying his importance in the big picture. Williams puts the Celtics over the top. They go from being very good to a great team. Assuming Tatum and Brown stay healthy, Williams is the next most important piece to the championship puzzle.

He’s a difference maker, a presence on both sides of the court, whether he’s swatting shots, pulling down rebounds, or finishing off thunderous dunks.

Having a less-than-healthy Williams during the postseason last year made winning much more difficult.

Mazzulla made note of Williams’ presence at the rim and impact on the game.

“I thought he did a good job protecting us in the paint, especially in the first half,” Mazzulla said following the game. “I thought he did a great job keeping our spacing and creating the right screens and angles for our guys. So when we have that vertical threat, that really helps us.”

With Williams healthy and playing at his best, the Celtics are arguably the best team in the NBA, although Milwaukee might disagree.

He’s the X-factor, because no one on the roster can provide what he does. No one else can dominate above the rim like he does. That’s probably why Mazzulla was so careful with him in the second half in what was, for the most part, a blowout.

“He was great for us on both ends, just active, flying around,” said White. “When you drive or have people come over, he’s just an outlet for us, whether it’s above the rim or bounce pass, whatever it might be.”

The Celtics have been extremely cautious with Williams, but if need be, they won’t hesitate about ramping up his workload, especially now during the postseason.

Said White: “He had a huge game. It’s nice to have him out there with us.”