PHILADELPHIA — The New Orleans Saints walked into Lincoln Financial Field on Sunday with a prime opportunity to bury the season’s slow start and begin building an identity around a remade roster full of newcomers.

The Saints fumbled that chance away.

Facing a Philadelphia Eagles team that seemingly entered Sunday’s game in disarray, New Orleans watched a close game evaporate in an ugly, mistake-filled second half that handed the Saints a disheartening 39-17 loss and left them with little room for recovery.

“You lose on Sundays in our league, and it’s a setback,” Saints coach Sean Payton said. “Each week, we’re playing an important game. It’s a team that we know pretty well in regards to what they do and what they do well, and we weren’t able to play well enough, with too many mistakes in a bunch of different areas. We didn’t do a good enough job coaching, obviously, and executing.”

For most of the first half, New Orleans (1-4) kept the avalanche at bay despite an ineffective offense that produced one early touchdown drive and then sputtered, unable to capitalize on two interceptions and two fourth-down stops by a defense under siege from Chip Kelly’s up-tempo offense.

“We knew the type of game this was going to be, the type of offense that Philly has,” quarterback Drew Brees said. “We knew offensively we were going to need to do a good job of possessing the ball and giving our defense a break at times, but we didn’t do that.”

Hemorrhaging yards, the defense kept coming up big in key situations, and with halftime approaching, the Saints got the ball back in a 7-7 tie with a chance to take a lead into the second half, trying to pull themselves out of an uneven first half by the single strand of a weakening thread.

Then the Eagles’ Fletcher Cox snapped that thread.

Cox beat the New Orleans offensive line and strip-sacked Brees, setting up a Caleb Sturgis field goal that gave the Eagles a 10-7 lead and effectively handed Philadelphia momentum for the rest of the game.

Protecting Brees has been a problem all season long, and it cost New Orleans dearly against the Eagles. Philadelphia, led by three sacks from Cox, sacked Brees five times and kept him under duress, forcing turnovers and disrupting New Orleans’ game plan. Philadelphia’s weakness is a secondary that’s susceptible to big plays; New Orleans tried to push the ball downfield, but the pressure disrupted Brees and instead created turnovers.

An Eagles front seven that has been largely ineffective took advantage of a Saints offensive line missing a pair of starters: left tackle Terron Armstead and right guard Jahri Evans.

“They brought a lot of games and pressures and stuff like that,” rookie tackle Andrus Peat said. “I’d have to see the tape, but I felt like they were using a lot of movement.”

New Orleans briefly stemmed the tide to start the second half, putting together a 10-play, 72-yard drive that tied the score at 10 early in the third quarter, but Philadelphia responded with a nine-play, 65-yard drive that Ryan Mathews capped with a 2-yard touchdown run.

At that point, the Saints offense still had a chance to start trading blows. New Orleans couldn’t come through.

Cox beat Peat, strip-sacking Brees again to hand the Eagles the ball at the New Orleans 13-yard line. Eagles quarterback Sam Bradford tossed a touchdown pass to Brent Celek on the next play and, from there, the avalanche only got worse.

Brees later tossed an interception, and new tight end Michael Hoomanawanui fumbled the ball away to give the Eagles another short field.

“This game should have been closer,” Brees said. “We turned the ball over four times; three of those gave them possession of the ball deep in our territory. So we basically gave them 17 points, and we only scored 17 points.”

Without any momentum on offense, the Saints defense finally capitulated, failing to make the kinds of key plays it had made in the first half. Philadelphia piled up 519 yards, overwhelming a Saints defense that was on the field for 34 minutes — nearly nine minutes more than Philadelphia had been averaging on offense.

In a game when New Orleans needed to score points — and had some opportunities in the first half — the Saints weren’t able to take advantage.

“It needs to be better,” Brees said. “We’re not scoring enough points. Regardless of what happens on the other side of the ball, at the end of the day, they say you have to score one more point than they do.”

New Orleans didn’t come close Sunday.

Now, the Saints have to find a way to regroup Thursday night against visiting Atlanta (5-0), which is riding high after an overtime escape Sunday against Washington.

There’s still optimism in the Saints locker room. But the missed opportunities are piling up as the rest of the NFC South takes advantage.

“After failing, we go on to (trying to improve to) 2-4,” defensive end Cam Jordan said. “That’s the goal, and that’s something that has to take place rapidly. We have to get better and get ready to play another opponent on Thursday.”