SAN DIEGO—Five days ago, the Denver Broncos were undefeated juggernauts defending their Super Bowl championship.

Today, they are on the brink of a perilous tailspin.

The Broncos once vaunted defense now insists on spotting its opponents a touchdown to get the game started.

Their head coach was forced to miss the trip because of a debilitating complex migraine condition.

Their offense, mediocre at best in winning times, now stinks, at least in the first half.

And the Broncos have now lost two in a row after they fell to the San Diego Chargers, 21-13, here Thursday on an otherwise near-perfect, mid-October evening at Qualcomm Stadium.

It could have been worse. After again failing to meet the appointed kickoff, Denver’s D rallied to limit the Chargers on four consecutive red zone trips, holding them to field goals each time.

“We’ve got to fix that,'' said Broncos' strong safety T.J. Ward. "We’ve got to get back to work. We’ve got to do something, play harder in the beginning or … I don’t know what it is, I can’t my finger on it, but it’s got to stop.’’

And the Chargers committed two turnovers that led to the Broncos’ first two scores.

Muffed punts by Charger returners gave the Broncos their first two scores. Denver went 0 yards on its first field goal drive in the second quarter. After a Charger safety put Broncos in a 21-3 hole in the fourth quarter, San Diego's Kenny Wiggins muffed Dixon's free kick at midfield. The Broncos went 51 yards to finally score a touchdown on a 5-yard pass from Trevor Siemian to Bennie Fowler.

It appeared the Broncos could have drawn to within, 21-16, with a chance at a 2-point conversion to make it 21-18, when a sensational catch-and-run for a 20-yard touchdown by running back C.J. Anderson was nullified by a holding penalty on left tackle Russell Okung and then a fumble by Demaryius Thomas. Okung was also called for holding on the safety.

After the game, Okung left on a cart with an IV and was placed in the back of an ambulance. He was taken to a local hospital with what the Broncos said was concussion-like symptoms, but it appeared overheating may have contributed to his condition.

The Broncos were called for 12 penalties in the game for a total 102 yards. And that yardage doesn't count the big plays the penalties nullified. Anderson had three big plays called back by penalties.

"You can't argue with the ref,'' said Broncos right tackle Donald Stephenson. "They don't always get it right, we don't always get it right. We can't put ourselves in a situation where a ref can make a difference.''

The Broncos did get a 46-yard field goal from Brandon McManus with 27 seconds remaining -- interim head coach Joe DeCamillis wisely taking the three points on first down to draw within one score. DeCamillis looked even smarter when Broncos' backup receiver Jordan Taylor, who had a costly fumble earlier in the game, recovered McManus' onside kick.

But Trevor Siemian's final play Hail Mary pass only reached the 6-yard line before it was knocked away by San Diego defender Dexter McCoil.

"I was kind of upset about that, I don't even think I got it to the end zone,'' Seimain said. "I probably threw it too soon. It's a shame because you get a chance to tie the game up there at the end and I don't even think I got it in there so that was my fault.''

Siemian may not have been able to get enough torque on his throw because of his left shoulder joint injury that forced him to miss the Broncos' game Sunday against Atlanta. After a slow start -- he completed 6 of 11 for a measly 39 yards in the first half -- Siemian found a rhythm in the second half through a controlled passing game. He finished 30 of 50 for 230 yards.

The Broncos are now 4-2 while the Chargers improved to 2-4. The loss snapped the Broncos’ NFL-record 15-game road winning streak against division opponents.

“We’re fighting but it’s been a little too late these past two games,'' said Broncos left guard Max Garcia. "We need to start fast. There’s been too many three and outs in the first half. We’ve been keeping our defense on the field and it’s really hurting us. We’ve got to be better at the end of the day.’’

The Broncos need to stop deferring.

For the second consecutive game, the Broncos won the coin toss and deferred, which meant kicking off to start the game.

Also for the second consecutive game, the Broncos’ starting quarterback was down, 7-0 before he took his first snap.

The Chargers were up 10-3 at halftime and it could have been more as they had 11 first downs to the Broncos’ 2, and 173 yards in offense to just 60 for Denver.

The Broncos have allowed a score on the opponents’ opening drive in five of their six games this year -- four touchdowns and a field goal. The exception was to Tampa Bay, which scored a touchdown on its second drive.

The Broncos have been outscored, 41-13 in the first quarter this season. Next time they win the coin toss, the Broncos need to realize they no longer have their 2015 defense.

"It's something we've got to address for sure,'' DeCamillis said. "It's been pretty consistent. I think whatever we can do to try and get that corrected will help us down the road. Obviously when Kube comes back I'm sure he's going to have a plan for that and we've got to follow suit with that plan. It's not just the players as far getting off to slow starts because I feel like as coaches we're not getting them going fast enough.''

With roughly 30 percent of the crowd wearing Broncos’ orange jerseys, the Broncos played as if their all-orange uniforms left them a step slow.

On Sunday, Atlanta quarterback Matt Ryan opened with quick, short passes to running backs to neutralize the Broncos’ vaunted pass rush and take an early, 7-0 lead.

On Thursday, it was San Diego quarterback Philip Rivers who got rid of it with quick, short strikes to his tight end duo of Antonio Gates and Hunter Henry, before connecting on a 5-yard touchdown to Henry, who beat Broncos cornerback Chris Harris Jr. on a slant route, for a 7-0 lead.

"They were sitting at 3 or 4 yards the whole game,'' said Broncos' edge rusher Shane Ray, who shared a sack with Jared Crick while Von Miller got the other. "Every rush we had to do had to be a power rush to push the pocket. We really didn't have many chances for our rush. They weren't going to let us get the edge, they were getting the release (on the pass) fast. Yhey don't want us to rush the passer.''

Rivers, whose Chargers have lost five in a row at home to the Broncos, came out smokin' in the first quarter, completing 10 of 12 passes for 119 yards. And his two incompletes were on the money.

“Was he smoking? All those little dunk and dink passes?'' said Broncos' cornerback Aqib Talib, still competing after he had taken his shower.

Rivers wasn't nearly as effective in the final three quarters and finished just 18 of 29 for 178 yards.

Whether it was rookie Paxton Lynch starting at quarterback against Atlanta or Siemian back from his left shoulder injury against the Chargers, the Broncos offense was behind before it began.

The Chargers marched 75 yards on each of their first two drives, although because they started their second one at the 6, they settled for a field goal and a 10-0 lead.

For most of the week, attention on the Broncos centered on the absence of head coach Gary Kubiak, who fell ill after his team’s first loss to Atlanta and was taken by ambulance to a local Denver hospital, the interim promotion of DeCamillis, who will now go back to his full-time job as special teams coordinator, and return of Siemian, right tackle Donald Stephenson and tight end Virgil Green from injuries.

Getting zero notice was the return of special teams standout Kayvon Webster from his hamstring injury. Playing the “gunner’’ position, Webster sprinted to down Riley Dixon’s first punt on the Chargers’ 6-yard line.

After the Broncos had to punt again off their second possession, Webster got close enough to Travis Benjamin to see the ball bounce off the Charger returner’s ankle. Webster tackled Benjamin away from the ball while Broncos’ safety Will Parks picked up the ball and ran 11 yards into the end zone.

The play was initially ruled Chargers’ ball, though, because the official didn’t initially see it bounce off Benjamin. Upon review, the Broncos got the ball but not the advance.

It cost the Broncos as their offense went nowhere and had to settle for a short Brandon McManus’ field goal.

Still, Webster’s play seemed to inspire the Denver defense, which held Rivers and company to a three-and-out on the Chargers’ next possession.

C.J. Anderson seemed to get the Broncos some decent field position with a 12-yard run but a holding penalty on left guard Max Garcia negated the gain and put the Broncos back to their 5.

Rather than risk a turnover, DeCamillis – or offensive coordinator Rick Dennison -- ordered three consecutive handoffs to end the half.

All five members of the Broncos' offensive line drew a major penalty in the game.

The Broncos now have 11 days off before they play former quarterback Brock Osweiler and the Houston Texans on Monday, Oct. 24 at Sports Authority Field at Mile High. The break will be much needed for many Broncos players -- and coach Kubiak.

Then again, the Broncos have 11 days to stew on their two-game losing streak.

"I wish I could have got a win for the city, got a win for the organization,'' DeCamillis said. "I'm sick to my stomach. It's sickening. Hate to lose, man, that's all. That's a bunch of war daddies in there that fight. I wish we could have done more to help them.''