If Brandon Weeden raises game, Cowboys could breathe easier

— -- The Dallas Cowboys went from playing without No. 1 receiver Dez Bryant to watching Jason Witten limp off the field (he later returned) to absorbing harsh news on quarterback Tony Romo, the player they could least afford to lose. With Romo's broken left clavicle requiring weeks to heal, we could reasonably declare the Cowboys finished, but that wouldn't be any fun -- and it might not be correct.

The Cowboys are 2-0. Both victories were in the NFC East. The rest of the division has a combined 1-5 record. This is a situation the Cowboys can manage if they can meet three criteria: backup Brandon Weeden is not horrible, the defense keeps making the kind of strides it has been making recently, and Romo returns for the finishing stretch. How much of a drop-off at quarterback can Dallas withstand while remaining in the playoff hunt? That's the question of the hour, and it's one we can answer using established methods. And if it's any consolation for Cowboys fans, ESPN's new Football Power Index (FPI) still has them as favorites to win the NFC East, even if Romo doesn't return until late in the season.

Projecting wins by QB

Let's assume Weeden starts the next 10 games while Romo recovers. Let's assume Romo would have played about as well in those 10 games as he played last season. And let's assume Weeden will play about as well as he played in his 21 NFL starts. That last assumption is the one that could be a problem for Dallas, because unless Weeden does better than he has in the past, the metrics say Dallas would win about three of its presumed 10 games without Romo, down from the seven of 10 they were predicted to win if had he stayed in the lineup (the exact figures were 3.2 victories with Weeden and 7.4 with Romo).

I'll explain the methodology below, but first let's consider what it would mean for the Cowboys if they went 3-7 with Weeden before Romo returned for the final four games. That would give them a 5-7 record heading into games against the Green Bay Packers (road), the New York Jets (home), the Buffalo Bills (road) and Washington (home).

That record sounds grim for Dallas, but this is only a projection. Romo could return earlier. Weeden could outperform expectations. Dallas' defense could surge in production once pass-rusher Greg Hardy makes his Dallas debut after his four-game suspension ends. And it's not as if the NFC East or NFC South divisions are loaded with wild-card contenders.

For Dallas to go 5-5 with Weeden instead of 3-7, the Cowboys probably would need his QBR to come in around 50 on the 100-point scale. Andy Dalton, Matthew Stafford and Jay Cutler were each a few points better than 50 in cumulative QBR last season. They were not great by any stretch of the imagination, but they did better than we might reasonably expect from a backup such as Weeden.

A quick word on methodology

When the Arizona Cardinals acquired Carson Palmer in 2013, I set out to project how many victories he might add, all else being equal. The method supplied at the time, which was created by ESPN's Production Analytics department, was pretty easy to understand. Total Quarterback Rating measures quarterback performance and correlates with winning percentages, so that teams with a 55.0 QBR in a game have won about 55 percent of the time over the years, and so on (see chart below, which includes all regular-season and postseason games since 2006, minus the late game Sunday).

To figure out how many victories a quarterback might add or subtract, we first calculate the difference between their average QBRs. Romo's average QBR for his 15 games last season was 74.0, while Weeden's average in his 21 career starts was 31.8. We next take the difference between those figures (42.2), convert it to a percentage expressed as a decimal (.422) and multiply that by the number of games Weeden would likely be the Cowboys' starter (10, in this case). The answer (4.2 games) is the difference in projected victories between Romo (7.4) and Weeden (3.2) over that span. Because QBRs correlate strongly with game outcomes, the projections are realistic.

Weeden has a 5-16 record as an NFL starter. That equates to a .238 winning percentage, right in line with what we'd expect for a player with a 25.2 QBR across those starts (that figure is cumulative, differing from the 31.8 average used in the previous calculations).

FPI still has Cowboys as favorites

ESPN's new Football Power Index (FPI) takes into account additional details specific to team and opponent strength. It's a much richer way of projecting game outcomes, and it likes the Cowboys' chances. The FPI, which was updated overnight, shows Dallas with a 48 percent shot at winning the NFC East, based on the projection that Romo would miss at least seven games (but possibly nine). The Philadelphia Eagles are next in line, with a 38 percent chance.

The chart shows the Cowboys' chances on a game-by-game basis, with and without Romo, for the nine games over the next 10 weeks.

If each quarterback's average winning rate held true over 10 games (Romo at 59.8 percent and Weeden at 43.8), starting the backup QB would change the team's record from 6-4 to 4-6 -- just a two-game difference. Those percentages will change as more information becomes available in upcoming weeks. For now, though, the Cowboys are in a position every other team in the division would envy.

Additional considerations for Dallas

Before last season, the Cowboys needed Romo to carry a disproportionate amount of the burden, covering for a poor defense and for an underused ground game (from 2010 through last season, no regular starting quarterback received less defensive support than Romo). But surging to a league-leading mark in team rushing percentage last season took pressure off Romo, helping to reduce his dropbacks by seven per game.

It's too early in the 2015 season to know how the Cowboys' formula for winning will evolve, but their defensive domination during a 20-10 victory at Philadelphia on Sunday was notable. This performance on defense produced 14.4 expected points added ( EPA), the Cowboys' second-best defensive effort in 68 total games since the start of the 2011 season. The Cowboys' top single-game performance since 2011 also was against the Eagles, in Week 7 of 2013.

Does what happened Sunday say more about the Eagles' offensive issues than where Dallas is headed on defense? The next several weeks will provide answers. There is some reason for optimism. This defense jumped to 20th in EPA last season, a nine-spot improvement in its first season with Rod Marinelli as coordinator. Linebacker Sean Lee is back from injury. Hardy is on the way.

On the down side, with Bryant and Romo on the sideline, Dallas' margin for error is exceedingly small -- and it's not even Week 3. First place in the NFC East has seldom felt so hollow.