ST. LOUIS -- A reeling and revamped defense will wash up Thursday night on the banks of the Mississippi River, likely carrying with it the 49ers' immediate fate.

The 49ers risk falling further off the NFC West pace if they can't regroup, clamp down and regain supremacy against the Rams, their long-time division rival.

Even without sack leader Aldon Smith and probably star linebacker Patrick Willis, defense should dictate Thursday night's outcome, much the same way the 49ers relied on that unit in 2011 to shepherd them out of the pre-Harbaugh doldrums.

The past two Sundays proved otherwise, as the 49ers sustained lopsided losses to the Seattle Seahawks (29-3) and Indianapolis Colts (27-7).

The San Francisco 49ers defense stops the Green Bay Packers’ Eddie Lacy (27) in the first half of their NFL game played at Candlestick Park in San
The San Francisco 49ers defense stops the Green Bay Packers' Eddie Lacy (27) in the first half of their NFL game played at Candlestick Park in San Francisco, Calif., on Sunday, Sept. 8, 2013. (Dan Honda/Bay Area News Group) (Dan Honda)

"Basically the last two weeks, we were in low-scoring defensive battles, and we were the first defense to crack," defensive coordinator Vic Fangio said. "When we get in low-scoring games, we've got to be able to hold the fort down and find a way to win those games with key stops, getting key turnovers and find a way to win them."

Thursday night's game could take it light on the scoreboard. The 49ers offense is in a so-called "dry spell," and the Rams offense is coming off a 31-7 drubbing in Dallas.

So tight was this matchup last season that both games went into overtime. They first tied 24-24 on Nov. 11 at Candlestick, when Alex Smith's concussion ignited Colin Kaepernick's rise. A Dec. 2 encore in St. Louis produced a 16-13 comeback win by the Rams in the final minute of overtime.

On a historical scale, the series is so tight that it's actually tied, 62-62-3, if you're counting San Francisco's 1989 playoff win. (The 49ers did count it in this week's pregame notes while the Rams did not.)

But how can the reigning NFC champion be only a field-goal favorite over a franchise that posted a 2-14 record only two seasons ago, a Rams club that featured the NFL's youngest roster on opening day this year?

The 49ers' offensive struggles are largely to blame, but their defensive teammates aren't pointing fingers, at least not yet.

"It's both sides of the ball," cornerback Carlos Rogers said. "The last two years, we've been creating a lot of turnovers, and they've been helping out the offense, and we've been stopping the run and leaving teams one-dimensional."

That defensive help hasn't been there the past few games, at least not after halftime.

Rogers and all the other cornerbacks are still searching for their first interceptions, a corps that likely will deploy Perrish Cox or Tramaine Brock as the third man in place of Nnamdi Asomugha (knee).

The 49ers' mantra of "stopping the run first" hasn't panned out the past two Sundays. While the Rams don't have an explosive running threat, second-year coach Jeff Fisher's history has Fangio still looking for a run-first approach.

Sam Bradford does have a pair of new, intriguing targets if he wants to air it out: tight end Jared Cook and slot receiver Tavon Austin.

Since Smith will be in rehab instead of rushing the passer, the 49ers will need increased production out of the likes of regulars Ahmad Brooks, Ray McDonald and Justin Smith.

Of course, the 49ers also will look for Smith's replacement -- or replacements -- to come through, as well. Former Cincinnati Bengals inside linebacker, Dan Skuta, is slated to start in Smith's place in what will be his first defensive snaps this season. Rookie Corey Lemonier could spell Skuta in nickel situations, as could Demarcus Dobbs.

"We need this win, so I'll do whatever it takes to do that," Skuta said.

Fangio sounded more concerned with penalties than personnel in terms of filling Smith's spot -- or potentially Willis' with Michael Wilhoite and Asomugha's with Cox and Brock.

"We've got to eliminate the pre-snap penalties and any post-snap penalty, neutral zone infractions, offsides, any personal fouls that would occur after the whistle," Fangio said. "Those are the ones that have to be eliminated."

  • No 49ers are listed as out on the injury report but six are questionable: Asomugha, Willis (groin), right tackle Anthony Davis (shoulder), tight end Vernon Davis (hamstring), safety Raymond Ventrone (concussion) and wide receiver Kyle Williams (rib, knee).

    For more on the 49ers, see Cam Inman's Hot Read blog at blogs.mercurynews.com/49ers. Follow him on Twitter at twitter.com/CamInman.

    THURSDAY'S GAME

    49ers (1-2) at St. Louis (1-2), NFL Network, KGO,
    5:25 p.m.