Leave it to the Celtics to add intrigue and drama to a season that was so promising just a week ago. Tuesday’s 13-point loss to the Heat in Miami was excusable on their ring night. And all of the excuses were used: too much time to prepare, chemistry not yet established, etc.

But Friday night’s 99-88 home-opening loss to the Bucks is reason to discard those excuses and concentrate on taking the first steps to becoming a passionate squad that plays hard all the time.

That was not the case Friday, as the Celtics expected the Bucks to play patsy because they are, well, the Bucks. But after taking a 10-8 lead, the Celtics allowed a 17-8 run and never came close again.

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The Bucks have a legitimate chance to win the Central with the recovery of the Bulls’ Derrick Rose and the injury to the Pacers’ Danny Granger, but they were supposed to be greener than the Celtics, playing their season opener. They took all of four minutes to shake out the kinks and figure out how to score easily on the Celtics.

In two games, opponents have shot 50.3 percent against Boston, including 47.1 percent on 3-pointers. The Celtics are just standing there on defense, and Rajon Rondo has been the main culprit, allowing opposing point guards to drive into the lane at will, on Friday night putting teammates in the unenviable position of trying to stop Brandon Jennings, or a big man off a pass.

After being passed up for a long-term contract extension, Jennings played with something to prove, and he dominated Rondo. Jennings is fast, and when allowed to roam free, very dangerous. Rondo often was caught behind screens or unfocused.

Rondo has been tabbed the team leader, and these are the times he has to set an example with his play and not his mouth.

“We have to trust one another defensively,” Rondo said. “It starts with me. I have to do a better job on the ball. We can say we [trust each other], but on the court it shows we’re not pulling in weakside [help]. There’s a lot of things.

“We’ve just got to start somewhere, and that’s the good thing about the league — we can start tomorrow.”

Coach Doc Rivers said he watched his team enter the game without a sense of urgency. That’s a disturbing sign, one that indicates the Celtics believe they can take supposed lesser opponents for granted.

But so far, the Celtics have showed they are inferior to the Heat and Bucks, who have attacked Boston at its foundation, its defense.

“We’re still trying to develop some chemistry with one another offensively and defensively,” Paul Pierce said. “Understanding the system, which is going to take some time. We’ve got time to get it together. Even though we made changes on defense, we have to do them and we have to do them the right way. It’s effort and communication, and tonight we just didn’t have either one of them. That’s the things that cause defensive breakdowns.”

The key word is effort, and that should cause concern for a team that prides itself on being strong in that category.

“Whomever is on the floor has got to play hard,” said Jeff Green. “I think collectively we’ve got to come together just a little bit more and just figure out a way. These games are going to be tough. Teams are no longer scared. We’ve just got to go out there and be the aggressor, just make plays and start it off on the defensive end.”

The Celtics are going to continue to struggle if Rondo allows penetration.

Jennings is a score-first guard, so when he adds 13 assists to his 21 points, the Celtics have little chance.

While Rondo has been recognized as a strong defender because he takes chances and makes steals, his primary weakness is the inability to play one-on-one. And that has to change.

“I’m going to try to come out and set the tone defensively [Saturday night against the Wizards],” Rondo said. “That’s all I can do. We’ve got to each look ourselves in the mirror and find a way to dig down and bring something to what we’ve been doing. Bring a little bit more to the game. I’m the point guard, I’m the first line of defense, I initiate the offense. I’ve got to be better.”

Rondo is learning leadership on the fly. And one of the first lessons is leading by example.