DENVER — Late in the game. Down six. Length of the field to go.

Usually, this is a situation in which Ben Roethlisberger shines.

This time, it was a moment he would rather forget.

Roethlisberger tried to thread a pass into a receiver and was intercepted by cornerback Tracy Porter, who returned it for a game-sealing 43-yard touchdown as the Denver Broncos beat the Pittsburgh Steelers 31-19 on Sunday night.

“My fault,” Roethlisberger said. “That loss is squarely on my shoulders.”

After the Steelers defense made a stand to keep it a one-score game, Roethlisberger and a banged-up offense had a chance to drive down the field to take the lead.

But it was so loud that Roethlisberger could barely hear the play. When he finally did understand what he was supposed to run, he tried to rush to the line of scrimmage to get it off in time.

In hindsight, Roethlisberger wishes he would’ve just called a timeout.

Also in hindsight, he wishes he wouldn’t have tried to force the pass to Emmanuel Sanders.

“I really thought he had a step on him,” Roethlisberger said. “I thought he broke out and I thought he had a step and I could get it to him. The guy made a good play.”

Porter did at that, making sure Peyton Manning’s debut in the Mile High City was a roaring success.

Up until that point, Roethlisberger was having a solid game. He was keeping Manning off the field with one methodical drive after another. Not only that, but the Steelers were converting third downs — 11 of 19 for the game.

That’s with a patchwork offensive line, too, after losing tackle Marcus Gilbert (left knee) and guard Ramon Foster (eye injury) in the first half.

“They weren’t third and short, either,” said Roethlisberger, who was 22 of 40 for 245 yards and two TDs. “We had a lot of third and long. That’s what Peyton kept saying to me (after the game), `You guys killed us on third down.’ We did. But at the end of the day it doesn’t matter when you lose the game.”

Manning overcame Pittsburgh’s 10-minute advantage in time of possession and helped the Broncos erase a 19-14 deficit.

He also threw the 400th touchdown pass of his career on a 71-yard strike to Demaryius Thomas. Manning joins Brett Favre (508) and Dan Marino (420) as the only NFL quarterbacks to reach that milestone. He also needed the fewest games to do so. This was Manning’s 209th game, while it took Favre 228 games and Marino 227.

“It’s just one game. I try to keep it in perspective,” Manning said. “I know how hard I’ve worked to get to this point. I know how much help that I’ve gotten along the way, from a lot of people. I’m very grateful for that. It’s special. I will definitely say it’s special.”

On Manning’s milestone TD, Thomas caught the ball between defenders and then simply took off. No one could catch him.

It was almost reminiscent of what happened last season between the two teams, when Denver ended Pittsburgh’s year on Thomas’ 80-yard TD catch and run from Tim Tebow on the first play of overtime in the AFC wild-card playoff game.

This time, though, there was no Tebow under center, just a four-time NFL MVP.

“Any loss is a bad loss,” defensive end Brett Keisel said. “Nobody likes to lose. You always want to start strong. We’ve got some things we need to get better at.”

The Steelers were without James Harrison as the Pro Bowl linebacker works his way back from minor surgery on his left knee. The loss of Harrison was a big blow to a stalwart Steelers defense that’s also missing safety Ryan Clark, who’s sitting out as a precaution because of a blood disorder that’s aggravated by altitude.

Running back Rashard Mendenhall (knee) didn’t play, either. Isaac Redman stepped in and had 11 carries for just 20 yards. His backup, Jonathan Dwyer, finished with 43 yards.

“I thought we did pretty well in a very hostile environment,” said Roethlisberger, who was sacked five times. “We’ll continue to grow and learn. We lost last year’s opener really bad and ugly in Baltimore.

“This one wasn’t as bad, but it’s bad for me. I hate that. I’m a competitor. It stings. But we’ll get better.”

Notes: Steelers K Shaun Suisham connected of field goals of 21 and 35 yards. ... TE Heath Miller and WR Mike Wallace hauled in short TD catches. ... Denver DT Ty Warren, playing in his first game since 2009, sprained an elbow in the first half and didn’t return.