The 49ers had five turnovers Saturday night, but the giveaway that provided the defining moment in a 33-14 preseason loss to the Broncos is clear.

Late in the first quarter, quarterback Brian Hoyer dropped back, cocked his arm to rip a downfield pass to wideout Jeremy Kerley and … had the ball slip from his grasp and fall behind him as he finished his throwing motion. The Broncos recovered tyhe loose ball.

“I went to throw it, and I was going to throw it hard, and, literally, I think my arm even went forward and then I’m looking around, like, ‘What happened to the ball?’” Hoyer said. “My quarterbacks coach asked me ‘When is the last time that happened to you?’ I think maybe a few years ago in practice it happened one time – I’ve never really had that happen in a game.

“It’s the worst feeling. You go to throw the ball, and you’re looking and all of a sudden, you look around and you have no idea where it is. It, literally, is the worst feeling that I’ve had as a quarterback.”

Head coach Kyle Shanahan chalked it up as a fluke play, noting he wasn’t about to tell Hoyer to grip the ball tighter.

“I didn’t need to talk to him,” Shanahan said. “It slipped,. It slipped out of his hands, so unfortunate for him.”

RB rotation: After they had impressive preseason debuts, rookie running backs Joe Williams and Matt Breida combined for one carry.

In fact, Breida didn’t play.

So what happened?

Shanahan said he wanted to give running backs Kapri Bibbs and Tim Hightower a chance to compete. Bibbs didn’t have a carry last week, and Hightower didn’t play.

“We’re trying to give them a chance to compete,” Shanahan said. “We see it in practice each day, but in games you want to see what guys do, breaking through those arm tackles, and see how they perform.

“We’ve got a tough competition at back. If you rotate them every series … you don’t give anyone a chance to show what they are. We try to do it that way. I’ve really tried to do it that way my entire career. You try to select which games you give guys a number of carries, so you have enough when it’s all said and done after four (games) to try to make a decision.”

The 49ers had possession for just over 21 minutes, which meant Bibbs (4 carries, 6 yards) and Hightower (3 carries, minus-1 yards) didn’t get much work. Hightower lost a fumble on his first carry.

Penalty parade: Shanahan was polite and relatively expansive, but a few of his answers were clipped after a loss that featured five turnovers and 11 more penalties.

The 49ers committed 17 penalties in their preseason opener, prompting a reporter to ask Shanahan what his answer was for the “continuous lack of discipline.”

“What do you mean by that?” Shanahan asked.

Told it was a reference to the 28 preseason penalties, Shanahan said, “You just keep working at it.”

Earlier, he did address the penalty problem.

“The penalties, again, I have to go in and look at,” Shanahan said. “It seems like there was bunch on both sides. We’re getting a lot called in the preseason. I’ve got to go find out about all of them. The ones I’m disappointed in are the pre-snap penalties. Those are ones I know we can do something about.”

Depth chart: Eli Harold started at outside linebacker after Ahmad Brooks assumed the role in the preseason opener.

Harold did not appear on the sheet, while Brooks played with the second team and had a quarterback hit.

“They’re competing to start at Sam linebacker,” Shanahan said. “We gave Ahmad that first game and Eli got this game.”

Up and down, again: For the second straight week, cornerback Rashard Robinson bounced back after a rugged start.

In the first four minutes, Robinson missed a third-down tackle that would have prevented a first down and was called for pass interference in the end zone.

He later was stride for stride with wide receiver Demaryius Thomas on an incomplete deep pass and made a strong open-field tackle on running back C.J. Anderson on third down to force a field-goal attempt.

“It’s a couple bad plays early in the game,” Shanahan said. “We’re still sticking with him.”

Quick study: Safety Jimmie Ward (hamstring) has been sidelined all summer, and Jaquiski Tartt missed time with a rib injury.

As a result, Lorenzo Jerome, an undrafted rookie from St. Francis University, has had a chance to make a strong impression. Jerome started Saturday ahead of Tartt.

“It looks like it,” Shanahan said when asked if Jerome has caught up to the speed of the NFL. “He’s had a solid camp. We’ve been banged up a lot at safety, so we’ve had to play a lot of guys. Jerome has taken advantage of his opportunity.”

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

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