The San Francisco 49ers and Chicago Bears are two of the top teams in the NFC, but that's with their respective starting quarterbacks.

Concussions have made at least one unavailable for Monday night's matchup at Candlestick Park between two of the league's toughest defenses.

With Jay Cutler out, the Bears will turn to Jason Campbell against a 49ers team that may end up starting backup Colin Kaepernick if Alex Smith can't go.

San Francisco (6-2-1) and Chicago (7-2) lead the NFC West and North, respectively, and seem destined for the playoffs. The 49ers are hoping to go one step further this time and reach the Super Bowl, while the Bears were stopped one game short the previous season.

Those aspirations have been dealt a few major blows, specifically to the heads of Smith and Cutler.

Smith was hit hard twice in last week's 24-all tie with St. Louis, leaving the game in the second quarter after overcoming blurred vision to throw a 14-yard touchdown pass to Michael Crabtree .

Smith practiced again on Saturday, however his availability is still uncertain. He still hasn't been cleared to play by doctors, and the 49ers might not get a final verdict on his status until hours before Monday's game.

"Alex is still going through the protocol right now," offensive coordinator Greg Roman said Saturday. "And that's in the doctor's hands. When we know, you will probably know."

The Bears believe Cutler's brain trauma came on a helmet-to-helmet hit by Houston's Tim Dobbins late in the first half of last Sunday's 13-6 loss. Dobbins was fined $30,000 earlier this week.

Chicago announced Friday that Cutler wouldn't play this week, meaning Campbell is slated to start.

The Bears brought in Campbell in the offseason just for a situation like this after Cutler broke his right thumb late last season and Chicago went 1-5 without him to miss the playoffs.

Campbell completed 11 of 19 passes for 94 yards in relief last week, his first extensive playing time since a road win over the Texans with Oakland in Week 5 of last year. He's started 70 games over the past six seasons, and has 70 touchdowns compared to 50 interceptions.

"He's comfortable in the pocket, comfortable calling the plays," center Roberto Garza said. "Look at his track record. That speaks for itself. There's a reason why he's here, and there's a reason why he's in the situation he's in.

"He probably should be starting somewhere. He's that type of player. Unfortunately, we're in this situation, but he's gonna step in there."

Campbell has completed 51.0 percent of his passes for 239 yards with one touchdown and two interceptions in two meetings with the Niners, both resulting in losses at San Francisco with Washington and the Raiders.

While Chicago may be turning to a veteran backup, the 49ers may be giving Kaepernick his first start.

A second-round pick out of Nevada in the 2011 draft, Kaepernick's most extensive action came after Smith went out last week. He went 11 for 17 for 117 yards, while rushing eight times for 66 yards and a touchdown.

He is approaching as if he will start against the Bears - the same way he prepares every week.

"Same reps as always. Nothing really changes," Kaepernick said. "I've always been out there preparing myself to start."

Kaepernick's running ability gives San Francisco a different dimension than Smith offers, but the Bears' defense is fifth in the NFL, and excellent against the run, allowing 92.3 yards per game.

Overall, Chicago gives up an average of 307.3 yards, and the unit leads the league with 30 turnovers and 19 interceptions. The Bears picked off Houston's Matt Schaub twice last week and held the Texans to a season-low 215 yards.

The Niners' Frank Gore will try to continue his success against the Bears. He's totaled 215 yards and a touchdown on 37 rushes while the teams have split the last two meetings.

San Francisco took the most recent matchup 10-6 at home on Nov. 12, 2009, when Smith and Cutler faced off on national television.

Another defensive struggle is likely since the 49ers are also among the best in the NFL, ranking third while allowing 292.1 yards per game - 196.8 through the air.

They've also shown the ability to get to the quarterback, getting eight of their 17 sacks in the past three games.

Kaepernick, 49ers whip Bears 32-7

SAN FRANCISCO (AP) -- As Alex Smith stood on the sideline with a concussion, San Francisco's potential quarterback of the future went to work - and fast.

Strong-armed fill-in Colin Kaepernick made all the right throws, looking every bit a capable NFL No. 1.

Kaepernick passed for 243 yards and two touchdowns in his first career start in place of the injured Smith, and the 49ers whipped the Chicago Bears 32-7 on Monday night in a highly touted NFC showdown that hardly lived up to the hype.

"It's everything I could've ever wished for," Kaepernick said. "It feels great just to be out there."

Kaepernick threw touchdown passes to Vernon Davis and Michael Crabtree , and Kendall Hunter ran for a 14-yard score as San Francisco (7-2-1) jumped out to a big lead by scoring on each of its first four possessions - with Aldon Smith wreaking havoc on the other side of the ball with 5 1/2 sacks.

Jason Campbell , the other quarterback in this matchup of backups for division leaders, threw a 13-yard touchdown pass to Brandon Marshall in the third quarter but was sacked five times and threw two interceptions in his first start since October 2011 for Oakland.

He faced fierce pressure all night, on the field for the Bears (7-3) as starter Jay Cutler recovers from a concussion suffered eight days earlier - just like Alex Smith .

After Kaepernick's stellar night on the big stage, there's certain to be chatter of a quarterback controversy for the NFC West-leading Niners. And anyone who knows coach Jim Harbaugh knows he's all about competition - at every spot on the field.

"I usually tend to go with the hot hand, and we've got two quarterbacks with hot hands," he said. "We'll make that decision when we have to make it."

Aldon Smith took over the NFL sacks lead with 15, passing Denver's Von Miller with 13, and recorded the second-best total in franchise history behind Fred Dean's six-sack day on Nov. 13, 1983, against New Orleans. Tarell Brown and Dashon Goldson each had an interception for San Francisco's stingy defense, which shut down Campbell, Matt Forte and Co. three years after the teams last met in a 10-6 49ers home win.

"I think I have a thing for night games, I love playing at night," Smith said. "I love playing under the lights."

Kaepernick, Aldon Smith and Hunter sure made general manager Trent Baalke look good for his selections from the 2011 draft class.

And reigning NFL Coach of the Year Harbaugh earned a key victory four days after his own health issue. The 48-year-old Harbaugh underwent a minor procedure for an irregular heartbeat Thursday.

The 49ers added a safety in the fourth quarter after a replay review. With 9:24 left, former San Francisco offensive lineman Chilo Rachal was called for intentional grounding out of the end zone, but Harbaugh challenged and the review showed Rachal's knee was down in the end zone before the ball left.

"Tonight was probably the worst nightmare. We just have to find a way," Campbell said. "It's one game that we lost. We have to pick it back up next week and try to get back on the winning side. Our goals and everything still sit ahead of us."

The soft-spoken, stone-faced Kaepernick went 16 for 23 with a 133.1 passer rating. He completed 12 of his first 14 passes with a 57-yard strike to Kyle Williams that set up Davis' 3-yard TD on the next play - and he already had amassed 126 yards passing by the end of the first quarter.

The 49ers led 17-0 on Hunter's early TD run in the second, quickly topping the 14.8 points the Bears were allowing per game.

Kaepernick threw for 184 yards in the first half alone - an impressive outing for the second-year pro selected in the second round out of Nevada.

"I think after the first drive I felt really comfortable with what they were doing and what we had in our game plan," he said. "I really wasn't too nervous. I've had a lot of time in this offense. My teammates were really supportive."

Frank Gore ran for 78 yards and David Akers kicked field goals of 32, 37 and 32 yards for the 49ers, eager to defend their home field a week after settling for a frustrating 24-24 tie against the division rival St. Louis Rams.

They outgained Chicago 249-35 in a lopsided first half.

Davis got his prime-time moment just how he loves it. Eager to get more involved in the offense down the stretch this season, the tight end had a team-best six catches for 83 yards.

"It felt like somebody took the handcuffs off," Davis said. "It sends out a message we're for real and we'll step up in big games."

Campbell was slow to get up after a hit by Ahmad Brooks with 6:06 left in the third quarter, not an encouraging sign as third-string QB Josh McCown started loosening up.

Chicago dropped into a first-place tie in the NFC North with Green Bay, which owns the tiebreaker after beating the Bears in Week 2. The Bears have lost two straight following a six-game winning streak.

Things were much less stressful on the opposite sideline, where Kaepernick chatted between series with Alex Smith - who was out of uniform and dressed in red 49ers jacket on a crisp, windy fall evening at sold-out Candlestick Park.

Smith, the 2005 No. 1 overall draft pick, was ruled out earlier Monday after being evaluated by team medical director Dr. Dan Garza.

Kaepernick completed an 8-yard pass to Mario Manningham on the first snap and hit Davis twice for 34 yards during the opening drive. But he overthrew Davis in the back of the end zone on third down and Akers kicked his first field goal.

Campbell had little chance as his offensive line was overmatched all night, spoiling his first start since last October and before a broken collarbone derailed his 2011 season and forced him to miss the final 10 games.

Forte was limited to 63 yards on 21 carries - not much better than his 41-yard day on 20 carries in the loss here in 2009, when Cutler threw five interceptions.

"I think we all let the team down at one point or another in the ballgame," Marshall said. "We're just taking turns."

San Francisco featured the opportunistic, ball-hawking defense this time after the Bears came in with an NFL-leading 30 takeaways and 19 interceptions.

"Coming in I think there may have been some questions about who may have been the better defense," Aldon Smith said. "We came out and proved a point tonight."

NOTES: The 49ers won their fourth straight Monday Night Football game, fifth in six and seventh of 10. ... San Francisco's 43 Monday Night Football victories match the Dallas Cowboys for the most. ... Bears WR Alshon Jeffery left with a knee injury in the second half.

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