49ers notes: Kyle Shanahan explains how Chargers' mistake clinched win

The play that ended the 49ers’ hopes Sunday against the Los Angeles Chargers was a mistake that caught everyone by surprise.

Quarterback C.J. Beathard was responsible for reacting quickly to get rid of the ball before the blitz arrived. But Chargers rookie safety Derwin James closed quickly and hit Beathard before he could unload the dump-off pass to running back Alfred Morris. The ball fluttered into the air and was intercepted late in the 49ers' 29-27 loss Sunday.

It was a rookie mistake, according to 49ers coach Kyle Shanahan said. That’s right: James is the one who made the error, and Shanahan said the safety wasn't just guessing.

“No, I know it was a mistake by them,” Shanahan said Monday, one day after the 49ers fell to 1-3 with the loss in Carson.

The 49ers' coaching staff knows Chargers defensive coordinator Gus Bradley very well. Defensive coordinator Robert Saleh worked with Bradley in Seattle and Jacksonville, where Bradley was the head coach. Shanahan offered Bradley a job as defensive coordinator last year, but Bradley turned it down to join Anthony Lynn’s staff.

“That was the first all-out blitz we’d seen by Gus Bradley in a long time, maybe ever,” Shanahan said. “It’s because it was a bust. Derwin is always the guy who blitzes unless the other safety (Jaleel Addae) is down, and that was the one time in the game the other safety was down and blitzed.

“C.J. was on it, saw the other safety and knew it. Derwin busted, and he blitzed also. So they had two guys blitz, which is a busted coverage. That’s what surprised us. That was a hell of a bust by Derwin.”

Reed solid in first start

49ers rookie D.J. Reed struggled last week in Kansas City when he was pressed into action at free safety after spending the entire work preparing to play strong safety and nickel back. But Shanahan said Reed did a nice job in his first NFL start against Philip Rivers and the Chargers.

Reed played all 68 defensive snaps with Adrian Colbert still bothered with a hip contusion. Colbert did not play on defense, but he was on the field for 16 special teams plays.

“I thought D.J. Reed did a good job in the game -- an improvement from the week before,” Shanahan said. “I think he’ll continue to get better with more reps.”

Shanahan said he anticipates Colbert to eventually regain his starting job, but it sounds as if there will be competition during the week.

“D.J. got his opportunity because of health reasons,” Shanahan said. “Sometimes all a guy needs is an opportunity. We’ll see how this week goes.”

Backup quarterback update

The 49ers do not plan to widen their search for a possible quarterback to add to the mix after bringing in seven players for workouts last week, Shanahan said.

“No, we just wanted to see those guys, see how they were, the guys that we had on our list,” Shanahan said. “They passed the test, and now we’ll see how it goes.”

Landry Jones, E.J. Manuel, Kellen Clemens, Tom Savage, T.J. Yates, Matt Simms and Kyle Allen were brought in for tryouts last Tuesday.

The team is set at quarterback on the 53-man roster with Nick Mullens serving as the backup to C.J. Beathard. The 49ers have no considered trading for a quarterback because the team envisions Jimmy Garoppolo and Beathard as the team’s two quarterbacks for 2019 and beyond.

If the 49ers did a make a move with a quarterback in the coming weeks, they likely would add a player for the practice squad, a source told NBC Sports Bay Area.

This ‘n’ that

Shanahan said he had no regrets about his strategy at the end of the first half to try to maneuver into position for a possible field goal. But the 49ers ran just 17 seconds off the clock with a three-and-out, then Bradley Pinion’s punt was returned 56 yards. The Chargers kicked a field goal on the final play of the half to tie the score at 17.

“I also know that if I had to do it over again, I’m going to try that,” Shanahan said. “At 17-14, I think we need to score some points in this game, and I think we had an opportunity to do it there.” ...

Wide receiver Victor Bolden, who returns this week off a four-game suspension for violating the league’s policy on performance-enhancing substances, will have a spot with the team. Shanahan said he was not sure whether it will be on the active roster or the practice squad. ...

Rookie receiver Dante Pettis is expected to miss multiple weeks with a knee injury, and Marquise Goodwin is questionable for this week’s game against the Cardinals with hamstring and thigh issues. Pettis and Goodwin play the same position. Bolden also is comfortable playing that spot.