49ers throttled by Broncos in exhibition debut at Levi's Stadium S.F.'s punchless QBs punctuate blowout defeat in stadium debut

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Oh, yeah, they played a football game in Santa Clara on Sunday afternoon.

With the pre-kickoff focus on traffic flow and trendy food - did you hear about those smoked jackfruit sliders? - the 49ers fell 34-0 to the Broncos in the first football game at Levi's Stadium.

The focus on the $1.3 billion patch of real estate, of course, was appropriate because this wasn't a count-in-the-standings game and the 49ers hadn't ushered in a new home in 43 years.

Still, those in the crowd who stayed in the stands, bypassing the chance to roam the new digs, might have trained their attention on a potential problem area: backup quarterback Blaine Gabbert, who entered after Colin Kaepernick played two series.

After a dud of a 49ers debut in the preseason opener Aug. 7, Gabbert fared only modestly better Sunday. He completed 8 of 14 passes for 40 yards with an interception, a stat line that translated to a 32.4 passer rating.

"I was up and down," Gabbert said. "I have to take care of the football. I'm working to be more consistent."

Colin Kaepernick (7) and Peyton Manning (18) chat after the Broncos defeated the 49ers 34-0 in the first preseason game at Levi's Stadium in Santa Clara, Calif., on Sunday, August 17, 2014. Colin Kaepernick (7) and Peyton Manning (18) chat after the Broncos defeated the 49ers 34-0 in the first preseason game at Levi's Stadium in Santa Clara, Calif., on Sunday, August 17, 2014. Photo: Carlos Avila Gonzalez, The Chronicle Buy photo Photo: Carlos Avila Gonzalez, The Chronicle Image 1 of / 25 Caption Close 49ers throttled by Broncos in exhibition debut at Levi's Stadium 1 / 25 Back to Gallery

Head coach Jim Harbaugh twice had skirted the issue when asked if there was an open competition for the backup-quarterback spot, but he struck a different note Sunday after his three reserves each had a turnover for the second straight game. Josh Johnson has lost two fumbles. Gabbert and McLeod Bethel-Thompson each have thrown two interceptions.

"In terms of a backup quarterback, it will be as simple as who doesn't turn the ball over," Harbaugh said. "They're turning the ball over. All of them have. There's no one to elevate. Whoever doesn't turn the ball over will be the backup quarterback."

Based on that criterion, the 49ers won't have a No. 2 if the reserves can't eliminate their miscues. In addition to their six turnovers, the backups have combined to complete 28 of 51 passes for 237 yards and post a 34.5 rating.

Last year, the 49ers signed Seneca Wallace after backup Colt McCoy struggled in the preseason. Harbaugh, however, didn't hesitate when asked if the 49ers would consider adding another quarterback to the roster.

"No, no," he said. "We have good quarterbacks."

On Sunday, Gabbert looked like a cautious quarterback, a trait best illustrated when he threw a 4-yard pass to closely covered tight end Derek Carrier on 3rd-and-10 from the 49ers' 16-yard line. Only two of his eight completions gained more than 6 yards, including a long of 12 yards, and he averaged 2.9 yards per attempt.

Gabbert did target Kassim Osgood about 20 yards downfield and was rewarded when cornerback Tony Carter was called for illegal contact. On Gabbert's other deep pass, though, Carter got payback when he intercepted a pass thrown behind wide receiver Quinton Patton and returned the pick 32 yards.

Harbaugh was asked if Gabbert's dink-and-dunk approach was part of the game plan.

"The throw was a deep ball that got intercepted," Harbaugh said. "I think Quinton could have looked earlier, but we're off. It's that kind of off. It's little and it's correctable, but it shows up. That's football. It punishes mistakes. It rewards execution and precision."

In the preseason, Gabbert has been imprecise. He has completed 44 percent of his passes (11 of 25), averaged 2.4 yards an attempt and posted a 17.9 passer rating. Gabbert's $2 million salary this season is fully guaranteed, meaning he would count against the 49ers' salary cap if he's released.

Last year, of course, there was also plenty of late-summer hand-wringing when McCoy posted a 52.6 preseason rating. However, Kaepernick muted that conversation by starting all 19 games, leaving McCoy to attempt one regular-season pass.

A year later, Kaepernick is working to ensure Gabbert will be ready in the event he misses his first game since he became a starter in 2012.

"I do everything I can to try and help him," Kaepernick said. "He is working hard and he is trying to make sure he is doing everything to go out and perform well."

Starting slowly Colin Kaepernick has played only three series in two preseason games. What he and the offense have done in those series: Points scored3 Total yards115 Passing6-for-10 Passing yards56 TDs-INTs0-0