First, Brian Hoyer stumbled, and then he fumbled, and his consecutive-play blooper reel neatly captured a disastrous performance for the 49ers’ first-team offense Saturday night.

In a 33-14 preseason loss to the Broncos, the 49ers quarterback directed an attack that left Levi’s Stadium aghast.

The results of the four first-half drives by the 49ers’ offensive starters: punt (three-and-out), fumble, interception and fumble.

It couldn’t have been any worse.

Or maybe it could have?

The 49ers, after all, were bumbling against a Denver defense that didn’t play seven starters. Imagine what might have happened if Broncos’ pass-rush extraordinaire Von Miller was pressuring Hoyer, who had issues with no defenders in his vicinity.

Late in the first quarter, Hoyer’s cleats appeared to get caught in the grass and he stumbled before throwing a wayward sideline pass in the direction of fullback Kyle Juszczyk. On the next play, Hoyer dropped back, cocked his arm and had the ball slip loose and drop behind him as he finished his throwing motion. The fumble was recovered by Denver.

It epitomized a first half that ended with the 49ers trailing 20-0 largely because of their four turnovers, one of which came on special teams.

At halftime, head coach Kyle Shanahan was asked on KPIX-TV about the “issues” he observed in the first two quarters.

“I think it’s pretty obvious,” Shanahan said. “You have four turnovers in a half and I’d expect it to be worse than it is right now. We put our defense in a crappy situation.”

It’s no time to panic after two exhibition games, but the 49ers certainly won’t celebrate the way they’ve started under their offensive-minded rookie coach.

The 49ers’ first-team offense has zero points on six preseason drives and a laundry list of errors.

On Saturday, the starters had two penalties (illegal shift, illegal formation), a lost fumble by running back Tim Hightower, an interception by Hoyer that was bobbled by wide receiver Marquise Goodwin, a fumbled pitch that running back Carlos Hyde recovered and a drop by tight end Vance McDonald.

“I think we just need to play cleaner, smarter football,” Juszczyk said on KPIX. “… We felt like we had a really good week of practice and we expected to come out here and translate that onto the field. Unfortunately that wasn’t the case. But, we’ll just get right back to it next week and we just know that we have to be cleaner.”

If there was any good news to come out of the messiness, it was this: Hoyer, the unquestioned starts, actually played well in spurts.

Hoyer completed 8 of 11 passes for 89 yards and had his back-luck pick. In the second quarter, his over-the-middle-pass was slightly behind Goodwin, who got both hands on the ball, bobbled it and had it ripped away by cornerback Chris Lewis-Harris.

The tone was for the nightmarish opening was established quickly.

The Broncos punted on their game-opening drive, but the kick hit 49ers safety Jaquiski Tartt in the back of his leg and was recovered by Denver at the 49ers’ 11-yard line.

Four plays later, after a pass-interference penalty by cornerback Rashard Robinson in the end zone, running back C.J. Anderson’s one-yard touchdown run gave the Broncos a 7-0 lead.

The 11-yard scoring drive marked the only points the 49ers’ defensive starters allowed in three series. The Broncos were forced to punt on their other two drives and averaged 2.1 yards on their 13 plays against the starters.

Denver scored all of its 20 first-half points off turnovers, which resulted in scoring drives of 11, 26, 12 and 53 yards.

“I thought the D did a solid job,” Shanahan said. “We put them in a bad situation four times.”

Eric Branch is a San Francisco Chronicle staff writer. Email: EBranch@sfchronicle.com Twitter: @Eric_Branch

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