Eagles should fire Jim Schwartz if Cowboys demolish them again. Cory Undlin, too. | Marcus Hayes

The road to the playoffs always went through Dallas, but the biggest roadblock hasn’t been Dak, Zeke, and Coop.

It’s been the Eagles’ defensive coordinator and his defensive-backs coach.

As much as Sunday evening will likely tell the tale of the 2019 NFC East title, it also should forecast the future for Jim Schwartz and Cory Undlin. Schwartz is the architect of a defense that has never played to its pedigree. Undlin was tasked to develop a young group of cornerbacks that is only getting worse. In 2017, the defense totaled 69 big plays: 38 sacks plus 31 forced turnovers. Last season when they went 9-7, that number dropped to 61, which is the current pace for the 7-7 Birds.

It sounds as if Eagles coach Doug Pederson is losing patience with his defense, and with his defensive backfield in particular.

He was asked after Sunday’s comeback at Washington specifically about his defense surrendering big plays for three consecutive weeks against bad teams that featured quarterbacks Ryan Fitzpatrick, Eli Manning, and Dwayne Haskins. Pederson replied:

“We have got to do better. Look at the tape, make the corrections, and we’ve got to tighten some guys up.”

He was asked Wednesday specifically about his secondary’s recent struggles. He replied:

“It all comes down to fundamentals and techniques. Once you leave training camp, you don’t leave the fundamentals and techniques in training camp. You’ve got to bring them with you throughout the course of [each] week. We’re at that stage of our game right now where you’ve got to continue to work those techniques. Those fundamentals.”

» READ MORE: The Eagles probably need to get healthier to match up with Dallas this week, and win the East

» READ MORE: Eagles place Kamu Grugier-Hill on injured reserve; Nelson Agholor, Jordan Howard are ‘status quo’ ahead of Cowboys game

Who "works those fundamentals?” Who’s in charge of “tightening guys up?” Coaches, that’s who.

They’d better be fundamentally tight Sunday, or the season will be over before you can say Merry Christmas.

The defense surrendered 37 points at Dallas on Oct. 20, when Amari Cooper had his third big game as a Cowboy against the Eagles. Ezekiel Elliott averages 163 yards from scrimmage in his five career starts against the Eagles, the most against any team he’s seen at least three times. In the teams’ meaningful games since 2016, when Schwartz arrived in Philadelphia, the Eagles are 1-4 against Dallas, and they have allowed an average of 30.5 points in the losses. They have lost three straight.

Dak Prescott will likely play hampered by shoulder, wrist and finger injuries, and if he shreds the secondary again — this time in Philadelphia, in December, with the season on the line — neither Schwartz nor Undlin should last into the new year. Face it: If Schwartz and Undlin can’t solve Jason Garrett and the Cowboys, then what’s the point?

Schwartz arrived with Doug Pederson in 2016 as an unofficial co-head coach, hired almost independent of Pederson, and held a mini-press conference at Pederson’s introductory press conference. The Lions’ head coach from 2009-13, Schwartz was an insurance policy if Pederson proved to be overmatched as a rookie head coach.

Undlin, a Chip Kelly holdover, predated both of them by a year, and has been credited with enhancing the careers of ... well-paid veteran safeties Malcolm Jenkins, and Rodney McLeod? Jenkins needed Cory Undlin the way Brett Favre needed Andy Reid as his quarterbacks coach in 1997, after Favre had won his second MVP award.

Whom, exactly, has this defensive staff developed? Nate Gerry? Meanwhile, the list of players who haven’t developed is damning.

» READ MORE: Eagles Pro Bowl picks: Zach Ertz, Brandon Brooks, Fletcher Cox, Jason Kelce, and Rick Lovato selected to roster

Top cornerback Ronald Darby, acquired via trade in 2017, has regressed. So has Jalen Mills, the green-haired seventh-round pick in 2016 who made his bones in 2017, the Super Bowl season. Avonte Maddox, a fourth-rounder last year, has played well at times, but he gave up two touchdowns last week at Washington. And those are the good ones.

The Birds took Sidney Jones in the second round of the 2017 draft to stash him — he was rated as a top-10 talent but was injured — then took Rasul Douglas in the third. They play like they were drafted three days ago, not three years. They should have developed enough to be starters. They have combined to play 34 of 237 defensive snaps in the past four games.

It’s not just the corners. Schwartz’s scheme is predicated on setting edge rushers wide, and, in 2017, he was given Derek Barnett, the 14th overall pick, who left Tennessee with Reggie White’s sack record. Barnett has just 12 sacks in three seasons — fewer than Washington’s Jonathan Allen, taken 17th; fewer than Atlanta’s Takkarist McKinley, taken 26th; and just one-half more than New England’s Deatrich Wise, taken 131st.

Of the four Eagles edge rushers who have taken at least 200 snaps, Barnett grades out a distant fourth, according to profootballfocus.com, which ranks him 80th in the NFL among edge rushers with at least 200 plays. Remember, it was Barnett’s expected development in Schwartz’s controversial “wide-nine” scheme that persuaded Chris Long to quit and spend the season producing podcasts.

In case you’re curious, Long had 11 1/2 sacks for the Eagles in his last two seasons, with one start, at the ages of 32 and 33.

Can Schwartz and Undlin save themselves? Probably. Win Sunday, secure a playoff berth the next week with a win over the Giants or a Cowboys loss, then win a playoff game against the 49ers, Seahawks, Vikings, or Packers.

Is it all their fault? Not really. Darby, Mills, Maddox, and Jones all missed time this season, but they’re healthy now. They’re just not playing like it. Past injuries don’t explain getting gutted in the second half against the Dolphins or in the first half against the Giants, then the Redskins.

Barnett, defensive tackles Fletcher Cox and Tim Jernigan, and linebackers Nigel Bradham and Kamu Grugier-Hill have been hobbled, too. None of that excuses only six turnovers in the last seven games. Nothing does.

Except bad coaching.