Jayson Tatum, above, and Jaylen Brown will be at the forefront for Boston this season. The Celtics felt like they had built a foundation for seasons to come when they added All-Stars Kyrie Irving and Gordon Hayward in the summer of 2017. Three seasons later both are gone, and Boston is now shifting its attention back on its youthful core. Mark J. Terrill/Associated Press

BOSTON — The Celtics thought they were laying a foundation for seasons to come when they brought in All-Stars Al Horford, Kyrie Irving and Gordon Hayward in back-to-back summers in 2016 and 2017.

Four years later all three are gone, the latest failed attempt by Boston to construct its first championship team since the Big Three of Paul Pierce, Kevin Garnett and Ray Allen led the Celtics to a title in 2008.

Now just three months after making a run to the Eastern Conference finals in the Florida bubble, Boston is shifting its attention and hopes to its youthful core as it prepares for a fresh start in the new season.

The Celtics are also trusting that, in his eighth year, Coach Brad Stevens can finally get the revamped roster over the hump after spending several seasons on the cusp of playing for the franchise’s 18th championship.

Stevens said he doesn’t feel like the recent departures are an indictment of what the team has been trying to build.

“I’d say everybody left for different reasons,” Stevens said. “Some of those reasons are layered. Some of those reasons are simple. We’ve had a unique situation here in that we’ve had some really good players that are older, and some really good younger players that we’re obviously building around.

“Our goal is to obviously be a little bit better than we’ve been.”

Much of the responsibility for helping the Celtics reach the next step will fall on the shoulders of Jayson Tatum and Jaylen Brown. The 22-year-old Tatum, who was drafted third overall by Boston in 2017, signed a five-year, $195 million contract extension last month. It came a year after Brown, 24, inked a four-year, $115 million extension to also remain with the team that drafted him third overall in 2016.

The only player in NBA history to produce 3,900 points, 1,300 rebounds, and 400 3-point field goals through his first three seasons, Tatum said his focus will remain unchanged a season after being selected for his first All-Star team.

“I don’t come in here and walk around saying that it’s my team. I don’t take that approach,” Tatum said. “Like everybody else, show up, get to work, and do my job to the best of my abilities and just go out there and play. I don’t feel like I need to go out there and say it.”

Brown said whatever spotlight exists for himself and Tatum is predicated on their ability to help raise the team’s overall level of play.

“I’m looking forward to the future. We’re here and we’ve been close. But we’re trying to get over that hump,” Brown said. “So we gotta win games and we gotta win games when it matters most. So I think that’s what we’re both focused on.”

WALKER’S STATUS

Along with Tatum and Brown, one of the biggest reasons the Celtics were able to have the success they did in the bubble was because of the play of point guard Kemba Walker.

Boston’s Kemba Walker will be sidelined until at least the first week of January after receiving a stem cell injection in his knee. Mark J. Terrill/Associated Press

In his first year after signing with Boston in free agency, Walker averaged 20.4 points and 5.8 assists during the regular season. But he nearly had his first playoff run with the Celtics derailed in July when he tweaked his left knee during workouts leading up to the resumption of the season.

He was limited during the seeding portion of the schedule before returning to average 19.6 points and 5.1 assists during the playoffs.

But he will be sidelined until at least the first week of January after receiving a stem cell injection in his knee.

Free agency addition Jeff Teague is an option to fill Walker’s spot in the lineup, as is veteran Marcus Smart, who started when Walker was injured last season.

NEW ADDITIONS

The Celtics also added some front court help, signing big man Tristan Thompson to a two-year deal. He will fill the role that opened after Enes Kanter left to sign with Portland.

Thompson was one of six forwards to average a double-double on at least 50% shooting last season. He not only adds championship experience from his time in Cleveland but also provides a toughness on defense that Stevens said is “one of his great strengths.”

“He plays physical. He plays with a high motor. He’s obviously got great experience. So I think that those things are all really positive,” Stevens said. “Anytime that you can add a guy that’s versatile defensively, that can also rebound on the other end of the floor, I think that helps you.”