SANTA CLARA >> Why didn’t the chicken cross the road?

Because 49ers coach Kyle Shanahan didn’t think he could get to the other side.

It was third-and-1 at the 49ers’ 39-yard line Sunday in Miami on their first possession. Yet Shanahan, as committed to the run as any professional coach I’ve covered in the last quarter-century, didn’t think he could get a single yard. In a game with no playoff ramifications.

Patrick Taylor Jr. came up short of the first down and the 49ers punted.

It’s not why the 49ers lost for the fifth time in six games, but it said plenty about how Shanahan views what has previously been a big part of his team’s success.

The 49ers lost to the Dolphins 29-17 and are out of the playoff race. There are a lot of reasons a team oddsmakers figured was No. 2 behind only defending champion Kansas City could fall so hard and so fast. Many of them will be legitimate.

Next up is potential No. 1 NFC seed Detroit (13-2) on Monday Night Football at Levi’s Stadium. The Lions are similar to the team that led the 49ers 24-7 at halftime in the NFC Championship Game but lost 34-31. The 49ers are faint copies of what they were last Jan. 28.

Regardless of what happens against the Lions, either the 49ers get back to their run-the-ball, stop-the-run roots or they become a pretender or perennial also-ran hoping to make the postseason.

Going into Week 17, 49ers opponents have run the ball 416 times. The 49ers have 411 carries. In 2023, when the 49ers finished as the top seed at 12-5, they were at plus-130 with 499 rushes and 369 rushes allowed. It was plus-115 in 2022 and plus-60 in 2021. When they went to the Super Bowl in 2019, it was plus-97.

If this is too much “inside football,” consider that Shanahan values rushing attempts as coin of the realm in terms of winning and losing. He loves his chances when the 49ers run the ball 30 times ... 35 times ... as many as it takes to convert third downs, control the physicality, clock and tempo.

The flip side is that while the contending 49ers were running the ball, their opponents were not — either because they were way behind or couldn’t make positive yardage against a stingy front.

Neither has happened in 2024, and it’s a big reason the 49ers are looking at double-digit losses instead of shooting for a high playoff seed.

With the 49ers in a major slide, there’s talk about Shanahan “re-inventing” himself. The reality is this: If Shanahan returns the 49ers’ personality as a hook-to-the-jaw run/pass team with the ability to take a punch, they’ll be back in the game as contenders.

Lately, the 49ers have been absorbing the punishment.

Until Green Bay’s Josh Jacobs gashed the 49ers for 106 yards on 26 carries (doing much of the damage in the first half) on Nov. 24, they hadn’t allowed a 100-yard rusher in 55 regular-season games dating to 2021.

The 49ers have now given up four 100-yard rushing games in the last five, with Buffalo’s James Cook getting 107 yards on 14 carries, the Rams’ Kyren Williams gaining 108 yards on 29 attempts and Miami’s De’Von Achane gaining 120 yards on 17 attempts, including a slam-the-door 50-yard run to seal the verdict.

There are plenty of reasons for the slide, otherwise known as “excuses” in the unforgiving NFL.

The 49ers lost defensive tackle Javon Hargrave early with a torn triceps. Middle linebacker Fred Warner has played with an ankle fracture. Running mate Dre Greenlaw didn’t come back until Week 15, and now may be done for the year. Safety Talanoa Hufanga has attempted to rebound from a torn ACL, an ankle sprain and then wrist surgery.

Shanahan recognizes what’s in front of him while stopping short of an alibi.

“I don’t think we’ve had the continuity in our defense to play as good as we have in the past with some of the linebackers that have been in and out,” Shanahan said Monday. “I think some of our run-stoppers that have been big for us on the D-line haven’t been as consistent.”

There are reasons to believe the 49ers can recapture some of their Shanahan personality in 2025. Running back Christian McCaffrey, the NFL’s leading rusher in 2023 and who looked in top form in Buffalo until a PCL strain sidelined him, could be back as a reasonable facsimile of himself.

Left tackle Trent Williams, whose road-grading abilities help transform a questionable offensive line into something formidable, is expected back at full strength. If everything works out, the 49ers will get a look at Isaac Guerendo in the last two games and then ponder what to do with restricted free agent Jordan Mason.

McCaffrey could wind up being a Hall of Fame candidate with a few more good years. Williams would be one if he retired today, which he’s not likely to do based on the contract extension he signed during training camp.

For as much as the NFL has gravitated toward passing in its rules, there are few things as satisfying as a successful run-the-ball-down-your-throat attack.

Philadelphia’s Saquon Barkley, Baltimore’s Derrick Henry and Green Bay’s Josh Jacobs all meet a nostalgic lead-back standard that has been lost. It must be torturing McCaffrey to miss it.

Shanahan believes deeply in balance — his 24-play pregame scripts always contain 12 runs and 12 passes — but he’s perfectly willing to keep running the ball as much as it takes to win.

When Jon Gruden took over the Raiders in 1998 and came advertised as a passing guru, one of the first things he said was: “You know what’s great about gaining 4 yards on first down? You can run it again.”

Don’t kid yourselves. A lot of coaches still feel this way because of what it means in terms of down and distance, tempo and imposing physicality.

The running game is the left jab the 49ers are missing on offense. And they’re taking too many jabs to the face on defense.

As the 49ers and Shanahan go about rectifying the ills of a season from hell, getting back in the gym figuratively to get back on the attack should be at the top of the list.