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The Dallas Cowboys benefited greatly from the element of surprise last season, with rookie quarterback-running back duo Dak Prescott and Ezekiel Elliott shocking opposing defenses throughout an unexpected first-place campaign in the NFC East.

This year, a cloud hangs over a franchise that is coming off a salary cap-strapped, acrimonious offseason. That element of surprise has disappeared, and the Cowboys look like a deeply flawed, highly beatable team.

That was exhibited Sunday in Denver, Colorado, where the Broncos dominated the Cowboys on both sides of the ball in a 42-17 victory.

The thin Dallas defense—one which in the offseason lost four defensive backs who played a combined 2,645 snaps in 2016, per Pro Football Reference—helped historically unimpressive Broncos quarterback-running back duo Trevor Siemian and C.J. Anderson resemble John Elway and Terrell Davis.

And against a defense that Football Outsiders ranked first in football in terms of DVOA (Defense-adjusted Value Over Average) last season, Prescott struggled to make big plays while Elliott had the worst game of his career.

It was by far the most lopsided loss of this new era in Dallas.

The Cowboys do have some valid excuses. Their young, young team will have days like these, and winning in Denver is never easy. But there is reason to be discouraged by Sunday's performance, because it didn't totally come out of nowhere and it could have been a lot worse.

When Zeke isn't Zeke, the Cowboys aren't the Cowboys

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There's evidence now that without Elliott on his game, the Dallas offense becomes remarkably ineffective. This was only the second dud-like performance of his young career, but the Cowboys have lost on both of those occasions.

The other came in his very first game last September, when they managed just one touchdown in a 20-19 loss to the New York Giants. That only score came on an abbreviated 35-yard drive after Giants quarterback Eli Manning threw an interception inside his own territory.

The second came Sunday, when both of their touchdowns again came on short drives that resulted from turnovers.

Elliott has rushed for 80-plus yards in 15 of his 17 career outings. But he picked up only 51 yards on 20 carries in that first meeting with the Giants and he had only eight yards on nine attempts against the Broncos.

In those two affairs, Dallas failed to drive more than 40 yards for a touchdown.

Relying on Zeke Elliott Record Points/game When he plays and rushes for 90+ yards 11-1 26.8 Other games 3-3 22.5 Pro Football Reference / NFL.com

The Cowboys don't show up against elite defenses

Offenses can be expected to perform at lower levels against stronger defenses, but the Cowboys are establishing a reputation as a team that struggle more than usual in those spots.

During the Prescott/Elliott era the Cowboys have now played five games against defenses that allowed 19 or fewer points per game last season. Their record in those games is 2-3, while their record in every other game they've played with Prescott and Elliott is 12-0.

Here's how they've fared in five games against Denver, Minnesota and the Giants versus 12 games against teams with defenses that weren't as highly rated:

Cowboys offense with Dak and Zeke Category Vs. top-6 scoring defenses Vs. the rest of the league Games 5 12 Record 2-3 12-0 Points/game 15.8 30.4 Games w/ 20-plus points 0 / 5 12 / 12 Yards/game 302.4 415.0 Yards/play 4.6 6.5 Prescott passer rating 72.6 116.5 Elliott yards/carry 3.7 5.4 70-yard TD drives 1 27 Excluding Week 17 of last season, when Elliott DNP (PFR/NFL.com -- rankings based on 2016 season)

Those are the types of defenses the Cowboys will eventually have to slay in January. If they make it that far.

Reason for concern

A lot of that is connected to the fact Prescott hasn't been able to consistently stretch out opposing defenses, and that conservative style often makes him vulnerable when facing high-quality opponents and/or when Elliott isn't there to draw attention away from the passing game.

Prescott completed just one of four passes beyond 15 yards in Week 1 against the Giants, and he again attempted just four such throws despite trailing all afternoon in Denver. He's just 3-of-8 on so-called deep pass attempts this season.

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He was bailed out in Week 1 by the fact Elliott ran well and the Giants were inept on offense, but the Cowboys' depleted defense performed poorly against Denver and Elliott didn't show up.

Under those circumstances, the result should have been predictable. It was the second-lowest-rated complete game of the 24-year-old quarterback's career, behind only a stinker against—you guessed it—the Giants last December, and just ahead of a mediocre showing against—yup—the Giants last September.

"But Brad," you might be screaming, "you can't expect Elliott to struggle like that very often and the Broncos have arguably the NFL's best defense."

And that's true, but consider that the Cowboys do still have to play Arizona, Kansas City, Atlanta, the Giants (again) and Seattle, and probably more elite defenses if they survive the regular season. And consider also that Elliott might still have to serve a six-game suspension, which could kick in as soon as Week 3.

With all of that looming and the element of surprise a thing of the past, the Cowboys look as though they'll have a hard time becoming the first repeat NFC East champions in over a decade.

Brad Gagnon has covered the NFL for Bleacher Report since 2012.

Follow @Brad_Gagnon