John Fox insists it's not just coach-speak when he says his unbeaten Denver Broncos have plenty of room for improvement.

He, of course, isn't referring to their stellar aerial game led by Peyton Manning, who Sunday will face a Dallas Cowboys defense that hasn't been very good against the pass.

Manning is off the best start of his career heading into his first visit to Dallas' $1.2 billion stadium. He leads the league in completions (117), completion percentage (75.0), yards (1,470), touchdowns (16) and passer rating (138.0) while not throwing an interception.

Manning went 28 of 34 for 327 yards and four touchdowns - two each to Demaryius Thomas and Wes Welker - before sitting out the fourth quarter of a 52-20 win over Philadelphia on Sunday. Denver set a team record for points.

The Broncos are averaging league bests of 44.8 points and 483.0 yards of offense. Last week's win was a team-record 15th straight in the regular season, and Manning extended his streak of throwing at least one TD pass to 27 games.

"He's efficient, man,'' defensive back Champ Bailey said. "And hopefully he gets better - I don't know how, but hopefully he does - because this guy's a prime example of what it takes to be a great quarterback in this league.

"I know a lot of quarterbacks wish they could do it like that.''

Despite Manning's success, Fox doesn't think his team is perfect despite the Broncos (4-0) getting off to such a solid start. He may be nitpicking, but he'd like to see more from the running game and better defense in the fourth quarter.

Denver averaged 86.0 yards rushing in its first two contests, though that has increased to 152.5 over the last two. The Broncos have allowed a touchdown in the final quarter of each game.

"It's still early in the season and I expect us to get better," Fox said. "People look at me funny when I say that, but there's still a lot of areas we need to improve at and can improve at."

The last time Manning faced the Cowboys, he went 36 of 48 for 365 yards and two touchdowns, but also threw four interceptions in Indianapolis' 38-35 overtime loss Dec. 5, 2010.

Dallas (2-2) is hoping it can have that kind of success against Manning this time around as well, especially after allowing 506 yards and 20 unanswered points in a 30-21 loss to San Diego on Sunday.

The Cowboys gave up 401 yards and three touchdowns through the air and are tied with Minnesota and the New York Giants for worst in the league with 10 TD passes allowed.

They rank 27th in the league in pass defense at 304.5 yards per game.

"Yeah, we made mistakes, but it's never going to happen again,'' defensive end DeMarcus Ware said. "People say never say never, but I feel like it's never going to happen again playing a game like that. That's just the way I feel about it.''

Coach Jason Garrett realizes that Manning is just a part of the challenge his defense faces this week. Welker is tied for the NFL lead with six touchdown receptions and Thomas is tied for sixth with 393 receiving yards.

"He's obviously as good of a quarterback who has ever played this game, and he's got a lot of weapons," Garrett said. "Not only do you have this incredibly good quarterback who has a great understanding of what he wants to do, but he's got some weapons to do it with."

Tony Romo is having a solid season with eight touchdowns and one interception while ranking fourth with a 105.0 passer rating. He's completed 72.4 percent of his passes - his career best in a full season is 66.3 percent in 2011 - after going 27 of 37 for 244 yards and two touchdowns last week, both to Dez Bryant.

DeMarco Murray ran for 70 yards against the Chargers and is third in the league with 356 on the season, but Denver is allowing a league-low 74.0 rushing yards per game. The Broncos are 30th with an average of 316.3 passing yards allowed, but Garrett believes that number has been inflated due to Denver holding large leads in each contest.

"If you analyze the statistics, they've been ahead in a lot of games and teams have had to throw the ball and not run it quite as much," Garrett said. "They're very stout against the run, and they're willing to give up those (passing) yards to win the ballgame."

Broncos weakside linebacker Danny Trevathan is questionable after suffering a knee injury toward the end of Wednesday's practice. Trevathan, a second-year pro, is second on the team with 30 tackles and 24 unassisted tackles and has a sack, an interception and four pass breakups.

Denver has won four straight meetings with Dallas, taking the most recent one 17-10 on Oct. 4, 2009.