Chargers know being favored Sunday means nothing

They are favored by double digits Sunday, in fact the biggest favorite in Week 4 of the NFL.

The Chargers fully understand what this means.

Nothing. If not less.

“Go back to the [Buffalo] Bills game last week,” cornerback Casey Hayward said. “Nobody expected them to win but the Bills. That’s all it took.”

Indeed, Buffalo went into Minnesota as a 17-point underdog and buried the Vikings by three touchdowns.

That marked the NFL’s largest upset in 23 years, meaning a San Francisco victory at StubHub Center, as a 10½-point underdog, would be routine by comparison.

For the Chargers, losing to the 49ers would be just the latest bit of early season frustration for a team expected to contend for the playoffs.

“We’re going to have to be ready,” Hayward said. “You don’t come ready to play in this league, you’re going to get hit in the mouth.”

The Chargers are 1-2, having lost to Kansas City and the Rams, two of the league’s most explosive offenses and two of the three teams that remain undefeated.

Their victory came on the road, in Buffalo, and was quite convincing until some second-half sleepwalking permitted the Bills to make it a game.

As coach of perhaps the most promising 1-2 team in the league, Anthony Lynn said losing to shiny opponents like the Chiefs and Rams doesn’t make it any easier.

“Not at all,” he said. “We never expected to lose those games.”

Lynn interviewed for the 49ers head coaching job before being hired by the Chargers. His defensive coordinator, Gus Bradley, was believed to be San Francisco’s first choice for the same position, which eventually went to Robert Saleh.

Lynn and 49ers coach Kyle Shanahan go back to Lynn’s playing days in Denver where Shanahan was a ballboy. All of which makes for a cozy reunion and some hearty pregame handshakes, until kickoff when both coaches begin a desperate attempt to avoid 1-3.

San Francisco enters having lost its starting quarterback Jimmy Garoppolo to season-ending knee surgery and with a struggling defense beset by injuries. A week ago, the Chiefs had five first-half possessions against the 49ers and punctuated each with an extra point, 35-10 about as sobering a halftime deficit as the NFL can offer.

Things are so discouraging that there already has been social media clamoring for Saleh to be fired, a fairly aggressive opinion seeing how the secondary has been losing healthy body parts seemingly by the play.

Only two teams don’t have an interception yet and San Francisco is one of them.

Hey, the 49ers are double-figure underdogs for a few reasons, even if the Chargers are certain Garoppolo’s replacement, C.J. Beathard, is more than capable.

“He can run with the ball,” Hayward said. “And he’s still making all the throws.”

Like the Chargers, the 49ers have struggled trying to generate a pass rush — good news for Philip Rivers as he prepares to make his 196th straight regular-season start. With 222 more yards, he’ll pass John Elway for eighth on the all-time list.

jeff.miller@latimes.com

Twitter: @JeffMillerLAT