The Celtics’ lack of defensive execution Tuesday night against the Miami Heat came as no surprise, because their adaptation to Doc Rivers’s adjusted system was spotty at best during the preseason.

While the Celtics hardly emphasized winning during exhibition games, they played defense consistently in only one — the nationally televised win over the Brooklyn Nets Oct. 16 when they were responding to losing their first three games against NBA teams, including a 32-point pasting by the Philadelphia 76ers.

On that night at the Barclays Center, the Celtics were motivated, playing the type of vicious full-court defense that caused turnovers. On opening night in Miami, they played timidly, as if this were the last postseason and they were afraid to fall too far behind by taking defensive chances.

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The Celtics veterans appeared to be unaware of the team’s increased depth. So often over the past two years, the starting five were fighting against Miami’s nine, hoping to stay close and perhaps use an unlikely fourth-quarter scoring surge by Paul Pierce or Kevin Garnett to prevail. But eventually the Celtics would fatigue and LeBron James, with his hulking strength and exponentially improving mid-range game, would beat them.

On Tuesday, the Celtics had enough firepower to compete with the Heat down the stretch and they likely shocked themselves with a late fourth-quarter run that sliced the deficit to 4 points before Chris Bosh took over with some key baskets to preserve Miami’s win.

Leandro Barbosa scored 16 points in 16 minutes and made a statement — after hearing talk of him being merely a cheerleader off the bench. The Celtics have lacked scoring punch off the bench since Eddie House left. Nate Robinson and Glen “Big Baby” Davis had their moments, but Barbosa and Jason Terry are experienced in that role.

The more encouraging sign was that the Celtics actually have a bench they can rely on. The newcomers, however, will take time to learn the defensive concepts and were unquestionably embarrassed at times by the precise Heat.

Miami’s three 30-point quarters infuriated Rivers, who dismissed the fact that his team scored 107 points.

“You can live with LeBron and [Dwyane] Wade making jump shots,” Rivers said. “But first play I think Ray [Allen] is on the floor, I think we leave him by himself in the corner. You would think we would know better.

“I really thought they took the fight to us most of the night. I thought they were the more physical team. I thought they were mentally tougher than us.

“When we made our runs, they kept their composure. When they made their runs, I didn’t think we were very good with our composure.”

The Celtics have 81 more chances to atone for opening night, but the most important lesson is that it will take time, perhaps 20 or 30 games, for the newcomers to learn from the veterans. And it will also take players such as Courtney Lee a while to realize that every game in Boston is important, just as Jeff Green learned coming from Oklahoma City.

Lee played Tuesday as if he were just trying to fit in, afraid to make mistakes, and in turn he was caught off guard or out of place by Miami’s attacking offense. All five of his fouls were touch fouls, doing nothing to slow down strongmen such as James or Wade. Hopefully he watched Garnett’s late-game fouls on Mario Chalmers and Rajon Rondo’s hook of Wade that was ruled a flagrant foul.

While this Celtics team may have a chemistry similar to that of the championship 2008 club, it was unrealistic to ask them to come together so quickly. Defense has to be the emphasis, because they showed Tuesday that they have scorers, even when the offensive execution is uneven.

“I just thought that was the story of the night, our defensive struggles,” Pierce said. “That’s not who we are, the way we defended tonight. We’re not going to be a team that’s going to give up 120 points.

“We gave up 30 points pretty much each and every quarter. We’ve got to establish our identity and who we are and what we’re going to be, make a decision, because that’s who we are. We’re going to be a defensive team. We’re going to stop teams from scoring. We’re going to keep them out of transition.

“We’ve got to be a team that shows resilience, be the enforcer out there. We pretty much was on our heels the whole night.”

That transition needs to happen soon. The Celtics have cost themselves dearly the past three seasons with painfully slow starts that hurt their playoff seeding. This team is talented and deep and seemingly on the same page with Allen gone and no internal beefs. So the next few weeks will be critical to the Celtics’ long-term success, and it will be on the newcomers to quickly learn the system.

“We’ve got a lot of new guys trying to understand our philosophy defensively,” Pierce said. “We had a lot of breakdowns. I thought we didn’t communicate, either. It’s all about communicating, talking.”