It's no surprise that a team with the NFL's highest-rated quarterback is still in contention for the playoffs with four weeks remaining in the regular season.

As the Eagles play Detroit on Sunday trying to stay even with the Cowboys in the NFC East and force a final-week showdown for the division title, the emergence of Nick Foles is what makes their continued relevance seem easy to understand.

What doesn't figure, however, despite the feel-good effects of four straight wins, is how a team ranked 31st in total defense could be in that same position.

There are several answers to the question, and the arrival of the Lions for the biggest game of the year will help reveal which of them are right and which are byproducts of the rosy glow that comes from a few timely wins.

If the Eagles defense can contain an offense that is deep in weapons and ranked second in the league, maybe the question won't matter any longer. In any case, they can't afford a stumble. Aside from the somewhat dicey hope of a wild-card slot, for the Dec. 29 finale against Dallas to mean anything the Eagles have to go into that game no worse than tied with the Cowboys, who hold the tiebreaker if the teams finish with the same record.

Detroit has some issues - the Lions lost at home against the Buccaneers two weeks ago, a singular achievement in the NFL this season - but it is still a team that can make you stumble. The focus here has been on the challenge of dealing with Calvin Johnson, the league's leading receiver, but for what sometimes ails the Eagles defense, the bigger problem might be dealing with running back Reggie Bush, who is having the best season of his career.

What the Eagles have been able to do on defense recently is employ a modified bend-don't-break philosophy that doesn't mind giving up yards as long as those yards don't lead to points. They are second-to-last in the league in net yards and first downs allowed, but are tied for 13th in points allowed by the defense. That's not great, but it's not 31st, either.

They've been able to do that because Foles doesn't throw interceptions to set up the opponents for easy scores, and because their special teams are also winning the field-position game. Opponents have a long way to go and - here's another key - the ones beaten by the Eagles haven't had offenses good enough to make those treks. Four of their wins came against offenses ranked in the bottom third of the league, another against a Green Bay team crippled by the loss of Aaron Rodgers, and two more against the Redskins, who still averaged 400 yards per game.

What bothers the Eagles as they have transitioned to a 3-4 defense are teams that employ misdirection and screens and, particularly, those with someone in the backfield capable of both running and receiving. The Lions are just such a team and they have just such a guy in Bush.

"You can get all caught up in Calvin Johnson, but Reggie is such a great running back and he can hurt you in multiple ways," linebacker Brandon Graham said. "You get out there and see him in person and he's a lot faster than what the film shows."

Bush is just the first of three straight combination backs the Eagles are scheduled to face, a stretch that continues with Adrian Peterson of the Vikings and Matt Forte of the Bears. All three are ranked in the top five for combined yards from scrimmage. The other comparable challenge the Eagles faced this year came from Jamaal Charles of Kansas City, and he had 172 yards combined rushing and receiving against them.

"Those guys cause problems because you usually have linebackers on running backs out of the backfield and that causes mismatches," safety Nate Allen said. "He's going to get the ball in his hands and what we have to try to limit is yards after the catch. The Lions do a great job of yards after the catch, so we have to have a lot of guys going to the ball."

The Eagles want to get pressure on quarterback Matthew Stafford to help the coverage, but the danger is that over-pursuit plays into the hands of the draw plays and screen passes that help make Bush so effective.

"When you play a guy like Reggie Bush, you have to honor twice as many plays as when you play a one-dimensional back," linebacker Connor Barwin said. "What isn't as obvious is that against a combo back, sometimes you end up defending empty sets [a backfield empty aside from the quarterback] with your base [3-4] personnel, which is not necessarily what you want to be doing. Then you have a lot of your bigger people outside the box covering people."

Doing so against Bush, who is both elusive and then capable of turning on 4.3-in-the-40 speed, is hazardous to a defense, as is bringing in extra coverage backs and then trying to stop a powerful running game.

"We've got to get a lot of hands on him, because he's quick, very quick," linebacker Trent Cole said.

This will be a good test for the defense, and a chance to answer some of the questions that linger. With a solid game against the Lions, the Eagles defense can't be accused of merely picking low-hanging fruit against poor offenses or somehow getting by despite allowing way too many yards. (It could also become another fortunate day in a series of those. Arizona's talented combo back Andre Ellington was out of last Sunday's game because of injury, and Bush missed practice time this past week with a calf strain that has him listed as merely "questionable" against the Eagles.)

Nevertheless, there are four games left and the Eagles can't afford a stumble. Or, at least, they can't stumble any more often than the Cowboys. It won't be a shock if they can keep going with the league's top-ranked quarterback. Keeping it going to the end without a sign that the defense is better than some of its statistics, however - that might be a little surprising.