For a change, they take it seriously

Fear this.

On Christmas Day, for the first time this season, the Lakers took the court without serene confidence in the fact they were going to win.

Since they beat their arch-rivals, the Boston Celtics, 92-83, this should be an invaluable experience in other games against teams they respect enough to fear.

Let’s see, there’s the Feb. 5 game at Boston.

And the Feb. 8 game at Cleveland.

They also play the Cavaliers on Jan. 19. That one is here, but if LeBron James and the guys, who have won 24 of 26, keep rolling, the Lakers might mount a major effort for them too!

Yet to be determined is whether the Lakers have undergone an actual attitude adjustment and will play hard all the time.

On the other hand, when you think of their fans booing them off the court at halftime in their last game here, when they trailed the New York Knicks by 15, followed by losses in Miami and Orlando and a rally from five points behind in the last 3:30 to win in Memphis, Christmas marked some turnaround.

The Lakers defended energetically, if not always well.

They showed that their physical advantages over the Celtics in size and depth matter.

Of course, for a while it looked as if there was still no advantage they had that the Celtics couldn’t overcome, as they overcame that 24-point lead in last spring’s pivotal Game 4.

If the Lakers didn’t want to admit it, and the Celtics were nice enough not to throw it in their faces (this time), the Lakers needed this game much more.

It was on their court. They were the ones who lost in last spring’s Finals, looking physically intimidated at the end.

They were the ones who had struggled in recent weeks, while the Celtics’ winning streak climbed to 19.

So, with the Lakers playing as hard as they could, taking a 10-point lead in the first half and an eight-point lead in the third quarter, they saw the Celtics tie it up.

Then, in a plot turn from last spring, the Lakers became the ones who gutted it out at the end, finishing with an 11-2 run, with Pau Gasol scoring seven of the points, totally shutting the Celtics down.

The Celtics grimaced and took it like the professionals they are, giving the Lakers credit.

Well, all but one Laker.

The exception was Andrew Bynum, who missed last spring’s series, whose modest line -- nine points, seven rebounds, two blocks -- doesn’t measure his impact.

Nor were the Celtics about to acknowledge it.

Does Bynum make a difference, Kevin Garnett was asked.

“No,” Garnett said.

Compared with last season’s team?

“No.”

It’s not a shock to run into another 7-footer?

“It’s not a shock because our defensive assignments are made if it was Chris Mihm, if it was Andrew Bynum, if it was Pau at the five [center] and Lamar [Odom] at the four [power forward],” Garnett said. “Our defensive assignments don’t change no matter who’s there. . . .

“Hell, it could be Shaq [O’Neal]. Our defensive assignments are what they are.”

Except they’re not.

Last spring, Gasol was single-covered and held to 14.7 points a game by the physical Kendrick Perkins. Now Perkins guards Bynum with Garnett on Gasol.

As great as KG is, he won’t beat Gasol up. In another departure from last spring, Gasol fought back at the end, going from a three-possession stretch in which he passed up two open jumpers and turned the ball over in the post, to his closing burst.

The Celtics, who pride themselves on execution, couldn’t get anything close to a good shot in the last two minutes.

Oh, and Rajon Rondo said they “argued among ourselves,” including one instance in which he and Ray Allen barked at each other so long, Paul Pierce came over and pushed Allen back toward the Laker he was supposed to guard to break it up.

“We argue like that all the time,” Garnett said. “Our communication is the strength of this team.”

If the Celtics can survive their own discussions and still play all out, it’s no wonder they’re so good.

For the moment, the Lakers are just glad to know they’re in the same league.

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mark.heisler@latimes.com