Eagles tackles Jalen Carter and Jordan Davis hit the wall last week. Can they rally at Dallas?

It began last Sunday as it had gone for most of the season.

Second-year defensive tackle Jordan Davis batted down Brock Purdy’s first throw. Haason Reddick and Fletcher Cox sacked Purdy on the second. Josh Sweat stunted, from the right end up the middle, and forced an incompletion on the third.

On the Niners’ next possession, rookie defensive tackle Jalen Carter bulled into the backfield and forced a 4-yard loss. Reddick then outfoxed Purdy, who executed a quick snap, by forcing him to step up into Carter’s arms and throw too quickly. Then Cox, who turns 33 next week, flushed Purdy to his right, where Reddick pursued and forced a throwaway.

And that, incredibly, was pretty much all the Eagles got out of the middle of their defensive line against the 49ers. After their six best snaps in a row since Nick Sirianni became head coach, the Eagles’ defensive line disappeared. Cox, Carter, Davis, Reddick, and Brandon Graham, all first-round picks, as well as Sweat, a $40 million sack artist, simply dissolved. Poof.

The 49ers then scored touchdowns on six consecutive drives. They converted eight of their last nine third downs. It was as breathtaking a reversal of play as I’ve ever seen.

Why? The tackles melted. The kids got tired and Fletch got old.

It’s not their fault, exactly. It happened because the NFL schedule is a grind. It happened because Davis and Carter, having taken big strides toward professional competence, still haven’t worked themselves into NFL shape. It happened because the other tackles couldn’t carry the kids anymore.

Sirianni told me that he’d recognized the kids’ workload issues against the Bills and he’d taken it easy on them at practices before the 49ers game. Sirianni added that he would work with the strength and conditioning staff during the week to determine what load each player could manage before Sunday night.

However it adds up, it might be the most important calculus the Eagles compute all season.

Will Carter and Davis be able to rebound in time for Dallas on Sunday night in Texas? Is there enough gas left in their 650-pound tanks? Will Tony Pollard see four-lane highways when he runs between the tackles? Will Dak Prescott get four seconds on his pass plays to find CeeDee Lamb?

With Cox still limited by injury, the play of Davis and Carter could determine the NFC’s No. 1 overall seed and the bye that comes with it. The Eagles hold a one-game lead over the Cowboys, 49ers, and Lions. If the Eagles lose at Dallas and three of the four teams finish with three losses — Dallas plays Detroit in Week 17 — then the 49ers get the bye.

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Carter and Davis look like they could use a bye right now. There’s no shame in that.

Carter is a rookie. He was playing his 11th NFL game, which is right about when rookies hit the rookie wall.

Davis essentially is a rookie, too. Last year he played the first six games, missed three games with an ankle injury, then played limited snaps the rest of the season.

It’s not just the number of games, it’s also the number of snaps. The Eagles entered the season carrying concerns about the fitness of both Carter and Davis, who left Georgia on exercise bikes. As such, the Eagles limited their participation.

Carter entered the game against the Bills two weeks ago averaging just 33 snaps, with a high of 51, and that was the game against Dallas just before the Eagles’ bye week, so he had plenty of time to recover. Davis was averaging about 30, with a high of 44, set in Game 6.

Then, the perfect snap-count storm gathered. Cox left the Bills game just before halftime with a groin injury. Milton Williams missed the Buffalo game with a concussion. The game went into overtime.

Davis more than doubled his average against the Bills, with 62 snaps. Davis played 71 snaps, which not only is the most by any Eagles defensive tackle this season, it’s more snaps than even Cox has taken in a game in three years. By the end of the first quarter Sunday against the 49ers, in the previous five quarters plus the five minutes of defensive overtime play, Carter and Davis had taken more snaps than they usually would take in two games.

They weren’t alone.

The Eagles defense as a whole lined up for 175 plays in the previous two weeks, the most of any team in a two-week span so far this season, according to the Fox broadcast last Sunday. They had done so against topflight opponents, in the Bills and Chiefs. They had done so with travel, and on short rest; the Chiefs game was in Kansas City on Monday Night Football. They had done so in wet, 40-degree temperatures, often in driving rain and wind.

Something had to give. It was the defensive tackles.

Cox did not practice the week before the game, so his conditioning suffered. His availability was a game-time decision predicated on how he responded to a bunch of pregame contortions while tethered to an elastic exercise band. After five strong games to start the season he’d begun to show his 12 high-mileage seasons. Understandably, Cox was a shell of himself in his 38 snaps last Sunday and was of little help.

Williams doesn’t get much recognition, but he is a key contributor. He started against the 49ers, his ninth start of the season, and he grades out higher than Cox, according to profootballfocus.com. But Williams wasn’t right against the Niners, either.

Really, none of them were.

The Eagles might be 10-2, but they had gotten consistently excellent play from only one sub-unit: the interior defensive line. Carter entered last Sunday rated the second-best defensive tackle in the NFL behind the Giants’ Dexter Lawrence. Davis has been the anchor in the Eagles’ run defense, which entered the Niners game ranked No. 2 in the league.

They were toothless after the first 15 minutes.

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The Niners offense is predicated on misdirection and timing, and, sure enough, Purdy used some quick-hitters and screens to defeat the D-line, but not always. Purdy had four seconds to throw when he hit George Kittle for the Niners’ first third-down conversion, three seconds on the next play, three-and-a-half on the next completion.

By the time Christian McCaffrey ripped off his first rushing gain, a 10-yard run, Davis was late off the ball and jogging. Purdy used four seconds again for Kittle’s subsequent 32-yard catch-and-run; Cox and Davis barely left the line of scrimmage. He had three-and-a-half the next time he hit Kittle for 25. Davis was a spectator on McCaffrey’s 9-yard run before the two-minute warning. Then four seconds to Brandon Aiyuk before McCaffrey ran past Carter for his 2-yard TD.

The halftime break changed nothing. The 49ers received the kickoff, Purdy went unmolested on big gainers to Kittle and McCaffrey, and that set up their third TD, and that was pretty much that, the third of six straight scores in a 42-point collapse.

The Cowboys are averaging 41 points at home. If the Birds remain soft up the middle, they’ll hit that number ― and more.