Celtics’ slow starts are frustrating and puzzling

The Celtics fell behind the Heat by 15 points in the first half Sunday.Winslow Townson/AP

On Friday, the Celtics roared back from a 19-point deficit to pull within 3 points against the Houston Rockets. On Sunday, they dug out of a 15-point hole to tie the score at 59 against the Miami Heat.

But that is where the turnarounds fizzled, as both comebacks turned into moral victories and both games turned into losses.

If the Celtics’ pluckiness was encouraging, it was tempered by the fact that big comebacks are inextricably linked to big deficits. Boston was outscored by a total of 54-36 in the opening quarters of those two games.

Several players blamed the tranquil starts on a lack of early effort or urgency, and this team simply cannot get by without effort or urgency.

“In the second half, we played hard on defense,” guard Avery Bradley said after the 83-75 loss to the Heat. “But it’s just too late.”

Added rookie guard Marcus Smart, “We have no more mulligans. We tend to front those deficits in the first half and dig ourselves a hole to come out of in the second half.”

The starts were deflating — if that word is still allowed in this city — and coach Brad Stevens was frustrated by the lack of early defensive intensity. He also found it puzzling. Although this team has visible flaws, effort has rarely been one of them.

“I didn’t see it at all on the West Coast trip,” Stevens said. “Even in Minnesota when we lost that game, we started off the game playing really hard defensively. We were really active, loud, communicating. I don’t know if that’s because our defense is in front of our bench or not. But we need to do it better.”

Rookie mistakes

Roxbury native and former UConn star Shabazz Napier has been in a slump, converting just 26.9 percent of his field goal attempts over the Heat’s last 10 games. But coach Erik Spoelstra said his conversation with Napier on Sunday was not about missed shots or errors.

“He has the confidence of his head coach, and I don’t even mind if he makes two or three mistakes from overaggression right now,” Spoelstra said. “But I want to see that type of attack mentality, and I’ll deal with those mistakes. But the passive play, that’s not what this team needs.”

Napier played 21 minutes in his TD Garden pro debut, registering 5 points, 4 rebounds, and 3 assists in the win over the Celtics.

Dawkins re-signs

The Celtics officially announced the signing of guard Andre Dawkins to a second 10-day contract. The former Duke standout did not play for the Celtics during his first 10-day stint, which ended Sunday, but he averaged 20 points and 2.8 assists in four games with the Maine Red Claws. If the Celtics wish to keep Dawkins after this 10-day deal expires, they will need to sign him for the remainder of the season.

Challenging times

The Heat and Celtics met in the playoffs for three consecutive years from 2010-12. But this is clearly a different time, as both are scuffling to remain in contention for the No. 8 seed in the Eastern Conference. “You have to embrace the different challenges,” Spoelstra said. “I know down there, Danny [Ainge] and Coach Stevens, they love the challenge that they have. We love the challenge that we have.” . . . On Tuesday, the Celtics will go to Madison Square Garden to face the streaking Knicks. Yes, the streaking Knicks. After a dismal 5-36 start, New York has won five of its last seven games, the latest being Sunday’s victory over the Lakers. New York (10-38) is seven games behind Boston in the Atlantic Division.