WALTHAM — Saying Wednesday night’s game against the Milwaukee Bucks is the most important regular-season game in the Brad Stevens era wouldn’t be outrageous. The Celtics have a precious opportunity to claim the No. 1 seed in the Eastern Conference with a victory at TD Garden.

A week ago, the Celtics were considered pretenders after losing by 23 points at home to the Cleveland Cavaliers, a game that then was considered the biggest of the Stevens era. The Cavaliers have stumbled since, losing three consecutive games, opening the door for the Celtics to claim the top seed with a victory.

LeBron James won’t be playing Wednesday for the Cavaliers against the Toronto Raptors and the Bucks announced that All-Star Giannis Antetokounmpo, guard Matthew Dellavedova, Khris Middleton, and Tony Snell didn’t travel to Boston, as Milwaukee already has its playoff spot clinched and appears indifferent about the seed.

The Bucks would have had to hope the Hawks lose their final two games, while they beat the Celtics to earn the fifth seed. Since the Bucks are guaranteed at least the sixth seed, they seem content to face the Raptors in the first round.

So there are no excuses for the Celtics, who would love to give primary players Isaiah Thomas, Jae Crowder, Amir Johnson, and Al Horford a good rest before the playoffs open, likely with a Game 1 on Sunday at TD Garden. Stevens has refrained from giving his players a breather, unlike most of the playoff contenders. That’s because he realizes how critical a top seed is to the organization.

If the Celtics earn the No. 1, seed, they are likely to face Chicago or Miami in the first round. They are 4-0 against the Heat this season and 2-2 against the Bulls, but those two losses were both razor-thin decisions in Chicago.

There is a sense of playoff uncertainty for the Celtics because this current group has yet to win a playoff series. They are 2-8 in the postseason under Stevens, including last year’s disappointing six-game loss to the Hawks.

So the Celtics feel as secure as they are going to be, even if there are few expectations they will make it all the way to the NBA Finals from outsiders.

“We have a little different mentality,’’ Crowder said. “I feel like we know we can win. It’s about us going out there and doing it each and every night. Our mentality has changed a little bit. I guess it’s the time we put in together, the progress about getting better as a unit has brought us to this point. It builds your confidence.’’

It’s understandable that the Celtics’ confidence might be delicate at this point because they have no postseason success from the core veterans, besides Horford, and his occurred with Atlanta. The inexperience is glaring but Horford may be critical to galvanizing the team with his 74 playoff games.

“We’re in a really good position,’’ Horford said. “It’s not like we’re fighting for our playoff lives. At the same time it’s nice opportunity for us to come out and get a win and finish out the regular season the right way.’’

Horford’s 2014-15 Hawks finished the regular season 60-22 and were the No. 1 seed in the Eastern Conference playoffs, in a position very similar to this year’s Celtics. Although the Hawks finished with the conference’s best record, they were hardly favored to win the conference title and were eventually swept in the conference finals by the second-seeded Cavaliers.

“In both cases, you have to just go out and prove what you’ve done,’’ Horford said. “People are always going to rely on experience. Cleveland has done it for so many years, or I should say LeBron has done it for so many years, that’s just the way it is, so why would you bet against that? For us, we’re not going to get caught in that. We’re going to keep working as a group and see what we can do.

“I feel really good about our team. What I’ve been saying is I want us to be playing at a high level at this point in the year, making sure we’re doing the little things on the defensive end.’’

Horford is a man of few words, but he uncharacteristically talked about his confidence in this team.

“These are the playoffs that I’ve looked forward to the most, probably since I’ve been in the league,’’ he said. “I feel very strong about this group. I think we have a special group. I can’t wait for us to get started.’’

The Celtics have a special opportunity to advance far in this postseason, with the challenges likely coming from Toronto, Washington, and Cleveland. The high level of uncertainty is understandable, but having home-court advantage throughout the playoffs should help the transition from contender to challenger.

But it begins with Wednesday. The Celtics have to take this game seriously and then prepare for what they plan to be a long playoff run. No longer are these guys upstarts, they could theoretically be the favorites.

Gary Washburn can be reached at gwashburn@globe.com. Follow him on Twitter @GwashburnGlobe.