DENVER — A wake-up call, or the sign of something more serious?

An unexpected 27-20 loss to the Chargers on Thursday night made Peyton Manning and the Broncos look average and placed all their regular-season goals in jeopardy.

“It humbles us and gets us back to the drawing board,” Denver defensive end Shaun Phillips said.

Once in the best position to capture the AFC West and home-field advantage through the playoffs, the Broncos (11-3) aren’t a sure thing for either anymore.

They have as many losses as their main competition in both races — Kansas City for the division and New England for home field.

Denver holds the tiebreaker against the Chiefs, but not the Patriots.

The Broncos finish the season on the road against Houston and Oakland, two teams with a combined record of 6-20. But after a home loss to San Diego (7-7), nothing feels like a sure thing anymore — even for the team that still averages 38 points per game.

The Chargers won this game by keeping Manning on the sideline.

Philip Rivers threw two touchdown passes to Keenan Allen and kept the San Diego offense on the field for 38:49.

Over a stretch that began late in the first quarter and lasted nearly half the game, the Broncos’ offense ran 13 plays and gained 13 yards.

“We didn’t play well, didn’t stay on the field, didn’t have the ball much and, when we did, we didn’t do much with it,” Manning said.

It helped that he wasn’t on the field much. It helped that Wes Welker wasn’t available. It helped that San Diego’s coach, Mike McCoy, was Manning’s offensive coordinator last year and knows how the quarterback’s mind works as well as anyone.

Still, Manning has shredded up the best-laid plans of pretty much every team he’s played this year. The thought that anyone could hold Denver’s 2013 offense to two touchdowns and 20 points seemed almost laughable. Until now. It’s highly likely that New England, Cincinnati and other possible playoff opponents will be devouring this game tape.