SAN FRANCISCO — 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 5½ 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.”