Kurtenbach: The 49ers use Christian McCaffrey like a rental car. He still looks early-season fresh.

There has been only one constant this season for the playoff-bound 49ers: Chaos.

Actually, check that. Make it two things: Chaos and Christian McCaffrey.

This season in Santa Clara has played out like a survival reality show. Injuries have turned the roster into a walking triage unit. Brock Purdy missed eight games. Four of the team’s five best non-quarterbacks were missing for Sunday’s game against the Bears.

By all laws of football, the 49ers should be spiraling.

And instead, the one guy who should be breaking down — McCaffrey — isn’t just surviving; he’s thriving.

Running back is the NFL’s ultimate position of attrition. It’s a job with the lifespan of a Spinal Tap drummer. Usually, by Week 17, a running back with a heavy workload looks like a used sedan with 200,000 miles on it — rattling, leaking fluids, and praying it can make it to the driveway.

McCaffrey? He’s playing a Benjamin Button campaign of pain. He looks bouncier, sprier, and more violent now than he did in September.

This, despite having 399 touches on the season — 46 more than any other player in football.

And yet he looks better than ever heading into January and the postseason.

How is that possible?

You could see the pep in McCaffrey’s step in the Week 16 win over the Colts, and you saw it again Sunday against Chicago. While everyone else is limping to the finish line, McCaffrey is sprinting through it like he just drank three Red Bulls. Behind an offensive line that has congealed into an elite unit in recent weeks, McCaffrey has hit his season-long runs in back-to-back weeks.

Against a Bears defense that was selling out to stop him like he was the only item on the menu, he posted 181 yards of total offense on 27 touches.

His jump cuts in the backfield looked summer fresh. He forced eight missed tackles on the ground. More absurdly, he racked up 112 rushing yards after contact. That was his most this season and the third-most of his career.

Sure, there are some schematic changes the Niners have made in the rushing attack in recent weeks. But Kyle Juszczyk’s “dash” motion reps don’t result in this kind of output.

No, it’s really just McCaffrey being an alien.

“I feel like we’ve been getting better this whole second half of the year,” Niners coach Kyle Shanahan said Sunday. “I feel like our guys are playing really well as a group right now, and when you play well as a group, usually the run game is what shows.”

Remember the panic from the analytics crowd and the local media about “managing” McCaffrey’s workload? So much for all that hand-wringing.

It turns out McCaffrey is like a cast-iron skillet — the more you use him, the better he gets.

And that bodes exceptionally well for San Francisco as they head into what is, effectively, a playoff game on Saturday against the rival Seahawks.

The No. 1 seed in the NFC West and the division title are on the line, but the Seattle defense isn’t the Bears or the Colts. No sir, this is a different kettle of thrown fish.

Seattle’s front six is ferocious, and they have dynamic playmakers in a chaotic defense designed to test every angle of Shanahan’s zone-blocking schemes.

To make matters worse, George Kittle and Trent Williams are massive question marks for the short week. McCaffrey could be down his two best bodyguards as he plays a third game in 13 days.

But why should that start mattering now? Don’t you know this team is beyond logic?

With all due respect to Purdy, Ricky Pearsall, Jauan Jennings, and the rest of the Niners’ in-and-out weapons, the formula for San Francisco’s success is simple enough to write on a single notecard:

If you have McCaffrey — a singular weapon in this league in its greatest era of offense — you have a chance.

And if you have this version of McCaffrey — the one who seems to be powered by fusion energy and spite for missing nearly all of 2024 — the sky is the limit.