SAN FRANCISCO (AP) - No matter the play or formation, Jason Campbell looked left after almost every snap. His eyes opened wide, and his head turned fast - rarely ever for a receiver.
Campbell's fill-in start for Jay Cutler turned into a painful outing Monday night, throwing two interceptions and getting sacked six times in the Chicago Bears' 32-7 loss to the San Francisco 49ers. Aldon Smith sacked Campbell 5½ times alone.
"Tonight was probably the worst nightmare," Campbell said.
Campbell completed 14 of 22 passes for 107 yards and a touchdown with Cutler out with a concussion. The former Raiders quarterback, making his first start since last October before a broken collarbone forced him to miss the final 10 games for Oakland, had little help on either side of the ball.
The 49ers limited Matt Forte to 63 yards on 21 carries - not much better than his 41-yard day on 20 carries in a 10-6 loss at Candlestick Park in 2009, when Cutler threw five interceptions.
"Definitely one of the toughest games I've been involved in in my career," Campbell said.
The Battle of the Backups became a one-sided affair.
Colin Kaepernick passed for 243 yards in his first career start in place of Alex Smith, who also was ruled out with a concussion. 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 as Chicago's offensive line collapsed.
In a matchup of NFC division leaders, Campbell's lone highlight came when he threw a 13-yard touchdown pass to Brandon Marshall in the third quarter. He faced fierce pressure all night for the Bears (7-3), who fell back into a tie with Green Bay in the NFC North.
Green Bay owns the tiebreaker after beating the Bears in Week 2. Chicago has now lost two straight following a six-game winning streak and, even worse, showing signs of regression.
"We have to leave this game as quickly as we possibly can, because we didn't do anything," Bears coach Lovie Smith said. "There's nothing good really to talk about."
Chicago certainly has no quarterback controversy now - except for maybe its backup role.
After Kaepernick's stellar night on the big stage, however, there's certain to be chatter of a permanent quarterback swap for the NFC West-leading Niners.
Aldon Smith took over the NFL sacks lead with 15, passing Denver's Von Miller with 13, after the second-best total in 49ers 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, Forte and Co. three years after the teams last met.
"We were playing our defense, stopping the run and then stopping the pass," Aldon Smith said. "I was just playing my game and getting after it."
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 Jim 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.
"It definitely wasn't our best effort, by no means," Campbell said.
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.
San Francisco outgained Chicago 249-35 in a lopsided first half.
"I think we all let the team down at one point or another in the ballgame," Marshall said. "We're just taking turns, whether it's me at wide receiver or the next man, we're just not making enough plays and not getting it done. It's frustrating."
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.
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.
San Francisco featured the opportunistic, ball-hawking defense this time after the Bears came in with an NFL-leading 30 takeaways and 19 interceptions. Instead, Chicago ended up with a minus-2 turnover ratio.
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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