When LeBron James left Cleveland again last summer, the East was open for the taking — and it was obvious who would take it.

The Celtics entered this season as the envy of every team outside of Golden State, possessing a dynamic, young core that took James’ Cavaliers to Game 7 of the Eastern Conference Finals. Boston had nearly ended Cleveland’s four-year reign atop the East without an injured Kyrie Irving, despite getting five minutes from the first year of the max contract of Gordon Hayward, who suffered a horrific leg injury in the team’s season opener.

Both All-Stars would be back this season. The young core would only improve. The next half-decade or so would be theirs.

Somehow, the Celtics wound up getting booed out of their own building before the next All-Star break, having blown a 28-point lead to the Clippers on Saturday night. Two days earlier, the Celtics surrendered an 18-point lead in a home loss to the Lakers.

“If [this is who we are], we won’t last long,” coach Brad Stevens said. “I think that I need to look at myself first and figure out what I can do to help that not happen. … If that means we have to play different rotations, call different things, start differently in quarters than we are; whatever the case may be. There’s an answer out there and we need to find it.”

Veteran Marcus Morris provided an explanation of why the most talented team in the East has the conference’s fifth-best record — an answer that only produces more questions.

“For me, it’s not really about the loss, it’s about the attitude that we’re playing with,” Morris said after Saturday’s loss. “Guys are hanging their heads. It’s not fun. We’re not competing at a high level. Even though we’re winning, it’s still not fun. I don’t see the joy in the game.

“I watch all these other teams around the league and guys are up on the bench — they’re jumping on the court, they’re enjoying their teammates’ success. They’re enjoying everything, they’re playing together, and they’re playing to win. And when I look at us, I just see a bunch of individuals.”

Several individuals have contributed to the problem.

Hayward, 28, hasn’t come back as the player he was prior to the injury, averaging 10.8 points, while shooting 31.7 percent from the perimeter. In his final season in Utah, Hayward averaged 21.9 points, while hitting 39.8 percent of 3-pointers.

Irving, 26, leads the team in scoring (23.6) and assists (6.9), but appears to be backtracking on his preseason commitment to re-sign with the Celtics (35-21) this summer, and hasn’t easily transitioned into a leadership role. Last month, Irving called out his younger teammates by calling James, apologizing to his former Cavs partner for creating problems early in his career.

“I did a poor job of setting an example for these young guys what it’s like to get something out of your teammates,” Irving said afterward. “Going forward, I want to test these young guys, but I can’t do it publicly. That was a learning experience for me, realizing the magnitude of my voice and what I mean to these guys.”

The young, homegrown core hasn’t carried as much weight as expected, either. Jayson Tatum, 20, hasn’t elevated to stardom yet and has seen his shooting percentages dip. Jaylen Brown, 22, and Terry Rozier, 24, also keys to Boston’s deep playoff run, have seen their minutes drop with the return of Hayward and Irving. Stevens has struggled to find the ideal balance in the team’s lineups and bring together a locker room that isn’t yet divided, but admittedly isn’t unified.

With two months until the playoffs begin, the Celtics have time to come together. The talent remains as impressive as any team in the East. But a run to the Finals has only gotten tougher, with Toronto, Philadelphia and Milwaukee all loading up at the trade deadline, while Boston was quiet.

The future is far more uncertain than it ever seemed it could be.

“It’s tough to win four straight and lose three straight,” Morris said a few weeks earlier. “I would be lying if I said we knew our identity because the identity of a good team don’t do that.”