The Dallas Cowboys looked plain bad on Thanksgiving Day against the Philadelphia Eagles. Worst game of the season in a 33-10 loss. Period. You know what? Leave it behind.

It’s the NFL. It happens. Ask the Eagles, who just a few weeks ago were clobbered by the Green Bay Packers, 53-20, and now lead the NFC East.

No, the real problem lies ahead for these 8-4 Cowboys, a legitimate playoff contender that has spent this season correcting some of deficiencies that led to three straight 8-8 seasons. That problem is December. In December, the Cowboys have stunk. And we’re not just talking about week 17.

 Want some positive? The Cowboys have a 5-0 record on the road, the best in the NFL. Three of Dallas’ final four games are on the road.

In the past three seasons the Cowboys have gone 5-9 in December. That includes a week 17 game in January 2012. In fact, it gets worse the deeper into the month you go. The Cowboys are 1-5 in their final two games of the season in that span. All were games that mattered in determining whether the Cowboys would go to the postseason or not.

This is the month in which we find out which playoff contenders are real contenders. This is the month we find out if the Cowboys are truly for real.

One bad game shouldn’t throw the whole season into question. But there are some definite question marks.

Start with the past five games: The Cowboys are 2-3. Then look at the home record: The Cowboys are 3-4.

Then look at those home losses to Washington, Arizona, San Francisco and Philadelphia. All four games saw the those teams get physical with the Cowboys offense, specifically Dez Bryant. In the four losses, Bryant has just 13 receptions, including two touchdowns. In the other eight games, he has 50 receptions, including eight touchdowns. When corners are consistently physical with Bryant, the Cowboys are less productive.

The Cowboys defense may have reached their ceiling. They’re trending downward on third-down defense, giving up a first down 45.5 percent this season. In the past three games, the Cowboys have given up a first down on third down 53.3 percent of the time. The Cowboys are failing to get opposing offenses off the field, and it’s leading to games like Thursday’s, when the Eagles scored on six of their first eight possessions to take a 30-10 lead.

The pass rush is still anemic. Tony Romo spent most of the game looking like the quarterback who started the season against San Francisco and not the quarterback who carved up the New York Giants a few days ago. His pass protection was, well, more porous than usual. That’s a nice way of putting it.

And everyone yelled at everyone else on the sideline on Thursday. There was enough “positive passion,” as Bryant once put it, to make you think the ship was sinking.

The ship isn’t sinking. At least not yet. But there is an abundance of evidence for concern as the Cowboys enter the final month of the season.

Want some positive? Okay, how about this: The Cowboys have a 5-0 record on the road, the best in the NFL. Three of Dallas’ final four games are on the road. Then look at 2009. The Cowboys were 8-4 after 12 games that season as well and went on to win three of their final four to finish 11-5 and win the NFC East.

If the Cowboys are going to deliver on the promise of their 6-1 start, they’ll have to solve what has become their traditional December downturn.