The Post's Jason Reid says after last night's loss to the Broncos, the Redskins need a win against the Chargers on Sunday if they want to do well this season. (Sarah Parnass/The Washington Post)

DENVER —The Washington Redskins did a superb sleight-of-hand routine on defense for close to 21 / 2 quarters here Sunday against the Denver Broncos. Even while facing quarterback Peyton Manning and playing without safeties Brandon Meriweather and Reed Doughty, the defense scored one touchdown, set up another and staked the Redskins to a 21-7 lead in the third quarter.

Then it all came tumbling down for the Redskins. The Broncos scored the game’s final 38 points and walked away with a 45-21 triumph at Sports Authority Field at Mile High.

“We did a little bit of everything well for 2 1 / 2 quarters,” nose tackle Barry Cofield said of the Redskins’ defensive effort. “And we did a little bit of everything bad for the rest of the game. Against a team like this, that will spell disaster.”

Said rookie cornerback David Amerson: “You just try to say, ‘Finish the game. Finish the game.’ But we didn’t do that and we’ve got to get better at that. . . . We were doing a good job and it just slipped away from us at the end.”

The Redskins were left to try to come to grips with a defensive performance that including forcing Manning into four turnovers, including three interceptions and a lost fumble. But it also included allowing Manning to throw for 354 yards and four touchdowns.

“I felt like they wore us down after a while,” linebacker Rob Jackson said. “They’re a good offense. You’ve got to play four quarters. They were running the no-huddle, catching us off balance, things like that. . . . I think we played well up until midway third quarter.”

The defense had done most of the heavy lifting to get the Redskins the lead. After the game was tied at 7 at halftime, Brian Orakpo recovered a third-quarter fumble caused by fellow linebacker Ryan Kerrigan, who knocked the ball from Manning’s hand and was credited with a sack. That set up the offense at the Denver 19-yard line and the Redskins cashed in with a touchdown run by tailback Alfred Morris. On the Broncos’ next offensive play, cornerback DeAngelo Hall intercepted a Manning pass and dashed 26 yards for a touchdown and a 14-point Redskins’ advantage.

“We went into the game and Haz (defensive coordinator Jim Haslett) told us we needed to get at least three” turnovers, Jackson said. “We got four. We came out. We scored in the second half. It’s just going back to playing four quarters. We’ve just got to finish the game.”

When Manning and the Broncos started rolling on offense, the points came in a hurry. Denver’s 38-0 finishing flurry came in the game’s final 23 minutes.

“It’s more frustrating not to get a win,” cornerback Josh Wilson said. “It don’t matter about all that other stuff. We just need a win. We had these guys where we wanted them and we weren’t able to come out and finish it off. . . . They had some screens that hurt us. They just made some plays.”

Said Cofield: “It was still early. Their offense is potent, can score a lot of points. In no way did we feel like the game was over that early in the game. But you also don’t expect to lose lopsided the other way after being up so late in the game.”

Asked whether the Redskins suffered defensive breakdowns or the Broncos simply upped their performance level on offense, Cofield said: “Probably a little bit of both, I’m sure.”

Hall had two of the Redskins’ interceptions. Safety Jordan Pugh, signed during the week after Meriweather was suspended by the league, had the other. With Doughty sidelined by a concussion, the Redskins rotated Pugh, rookie Bacarri Rambo, Jose Gumbs and converted cornerback E.J. Biggers at the two safety spots.

“We knew they were an explosive offense,” Pugh said. “When you have Peyton, you know points are always there. We just tried to do what we could to hold them down, to stop points. We just had to stop them when the opportunity was there. We just have to finish on a better note.”

Said Amerson: “It’s frustrating just to see it all go downhill like that.”