With both teams atop their conferences, today's game is an actual event, as opposed to made-for-TV Christmas hypes like Shaquille O'Neal vs. Kobe Bryant I, II and III that paired the Lakers and Miami annually from 2004-2006.

Few rivalries in any sport can match the fervor and appeal of Lakers vs. Celtics. It's just as it was, with Celtics fans chanting "Beat L.A!" during Tuesday's win over Philadelphia.

Celtics fans hate the Lakers so much -- and Lakers fans so hate the Celtics -- that both live to see the other team lose, no matter who beats them.

Boston fans first chanted "Beat L.A!" in 1982, to the Philadelphia 76ers who were in the process of upsetting their team in Game 7 of the Eastern Conference finals in Boston Garden.

"Classic Boston," Kevin Garnett said of the Tuesday chant. "That's what's up."

Garnett has been in Boston for one season plus 29 games. When you're a Celtic or Laker, you're a Celtic or Laker all the way.

It took a miracle, or two of them, to re-start the ancient rivalry, with the Lakers coming off a No. 7 finish, the Celtics coming off a No. 15 finish two seasons ago.

Both were just trying to survive after that 2006-2007 season, with Kobe Bryant's days of rage, the Celtics' brass recoiling on camera at dropping to No. 5 in the lottery, and Paul Pierce close to asking to be traded.

Nor was it certain after their remarkable march to last spring's Finals that they could count on seeing each other, same place, same time, since one looked vulnerable.

Six months later, it's hard to remember that was the Celtics with their thirty-something Big Three and thin bench, even before they lost James Posey and P.J. Brown.

Coming off a harrowing 26-game postseason with Game 7s against Atlanta and Cleveland, there was no expectation the Celtics could come close to last season's 29-3 start.

So much for expectations.

Meanwhile, the Lakers, who were supposed to be: 1) a rising juggernaut, and 2) bent on revenge, spent recent weeks congratulating themselves on their record.

With notions of greatness and a 70-win season fading, Lakers fans actually booed their team off the floor at halftime of the New York Knicks game when it trailed by 15.

Suddenly, the question was: Who are you and what did you do with the Lakers?

The team wasn't just sloppy, it wasn't playing hard. Worse, the Lakers showed no sense of urgency, giving out reassuring "We're 21-3" updates.

Two weeks ago, Bryant acknowledged he had deliberately chilled out ("No, I haven't kicked over any coolers"), a tipoff to what was going on.

Because this was 180 degrees from Bryant's take-no-prisoners nature, and because the same sentiment was echoed by others who never said such things, like Derek Fisher, it was almost certainly coming from Coach Phil Jackson.

Jackson thought their struggle, especially on defense, was a result of their youth and just needed time to work out.

Time apparently ran out on the just-concluded 2-2 trip, with its meeting in which players and coaches had a frank and open discussion, as they say in diplomatic circles, not to mention a loud one.

Not that the Lakers are where they need to be, or even close.

The Celtics are already a great defensive team, with a scheme they've mastered and 100% commitment.

The Lakers have a new defensive scheme they haven't expended a lot of energy learning.

They finally played hard at the defensive end in New Orleans, keeping Chris Paul from running pick-and-rolls and holding the Hornets to 87 points.

Nevertheless, Lakers, who were supposed to be funneling Hornets to teammates, could still be seen funneling them to the hoop. Andrew Bynum yelled at Sasha Vujacic after one breakdown and Jackson yelled at Lamar Odom after another.

Of course, the Lakers might still be kicking back if they weren't facing you-know-who on Christmas.

As always, both teams have learned to hate the other for causes too numerous to name, like Jackson's memory of Celtics fans rocking their bus after Game 6.

"Maybe it was kind of like being bused as a kid in the '70s in Boston, if you remember that," Jackson said.

If you don't, he's referring to the riots in South Boston over integrating the schools.

Rocking the Lakers bus is an old rite of Celtics fans, who did the same thing after the 1984 series. If the Lakers are back next spring, they may have to consider alternate transportation, like evacuating everyone by helicopter from the roof of the TD Banknorth Garden.

In the meantime, this game will do. This is what the Basketball gods made Lakers and Celtics for, to test each other.

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mark.heisler@latimes.com