ON THE LAKERS : DENVER 105, LAKERS 79 : Real rivalry? Lakers don’t know half of it : Jackson won’t admit it, but Nuggets look like serious threat in West. And L.A. looks awful in second-half collapse.

Phil Jackson might want to rethink this thing.

Before Friday’s game, long before the Denver Nuggets thumped the Lakers, pounding them in practically every category in front of an amped-up crowd, Jackson said he didn’t consider Denver to be a Western Conference rival.

Instinctively, he said the Lakers’ main rival was . . . Sacramento?

Oh, boy.

Hold the cowbells, forget all those cow-town jokes Jackson uttered earlier this decade. The Lakers apparently have a new adversary in the West, in case a 105-79 Nuggets victory didn’t prove the point at the Pepsi Center.

San Antonio is always looming in the background, and the Lakers haven’t won in Portland in forever, but recent history suggests the Nuggets and Lakers will be a matchup to monitor.

The Lakers were pitiful Friday, shooting 35.2% and more or less sitting out the second half. In fact, it was the worst half in their history. They scored only 23 points, somehow outdoing their 24-point effort in the second half against Memphis in December 2004.

Along those lines, the Lakers had only two assists in the second half, their first coming with 4:45 to play. The Lakers’ media guide doesn’t list their all-time low for assists in a half, but if it’s not a team record, it’s close.

“No one stepped up the second half and played ball,” Jackson said.

A 58-56 Nuggets halftime lead turned into a rout after an unpredictably one-sided third quarter dominated by the Nuggets by a head-scratching 29-8 margin.

Nuggets fans, perhaps recalling how their team was eliminated on this very court by the Lakers in last season’s Western finals, gleefully chanted “Go home, Lakers” in the final minutes.

The Lakers did, gladly, on a night when snow fell outside and the sky seemed to fall on them in the arena.

Kobe Bryant had 19 points, none in the second half. Derek Fisher was scoreless in 23 minutes. Andrew Bynum had 19 points and 15 rebounds, the only positive line of note for the Lakers, who made four of 20 shots in the third quarter and a deplorable 10 of 44 in the second half.

It wasn’t a great 30th birthday for Ron Artest, who had 11 points on four-for-11 shooting and battled foul problems while guarding Carmelo Anthony, who had 25 points.

But it was a great night for the Nuggets, who rolled up their third-largest margin of victory over the Lakers since joining the NBA in 1976.

“They just beat us straight up and down,” Bryant said. “It’s probably developing into [something]. We’ll probably see them in the playoffs again and that will be three straight years.”

The Lakers were short-handed, no Pau Gasol for a ninth consecutive game because of a strained right hamstring, but even he might not have changed the outcome Friday.

The Lakers fended off the Nuggets in a testy playoff series six months ago, and, more recently, the teams were called for six technical fouls and two flagrant fouls in an exhibition game a few weeks ago.

Conflict made an appearance in the third quarter Friday when Kenyon Martin tied up Lamar Odom for a jump ball, leading to a few exchanged words.

Jackson, however, said beforehand that the Lakers considered the Nuggets only a top-four team in the West. Nothing more, nothing less.

“We told the players it was much more important to [Denver] than it was to us, and you guys have got to meet that challenge,” Jackson said after the game.

The Lakers were pummeled in the third quarter, one of their most lopsided quarters in recent memory. It only got worse for them.

They were historically bad in the second half, in a season when there had been whispers of chasing the Chicago Bulls’ legendary 72-10 record in 1995-96.

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mike.bresnahan@latimes.com

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