If the Giants do not make the playoffs, they will rue the final 83 seconds of the first half and first 159 seconds of the second half Sunday as the point where a promising season slipped off the rails.

The defense allowed three scores in that decisive span and the special teams committed a turnover as a comfortable 10-point lead disappeared in a hurry. 

By the time the Giants turned to J.J. McCarthy after allowing 34 straight points and McCarthy turned in a magic fourth quarter, it was too late to prevent a 34-27 loss to the spiraling Saints at MetLife Stadium.

With Cam Skattebo benched due to ball-security issues, the Giants didn’t have enough playmaking until McCarthy led the offense to 17 unanswered fourth-quarter points and forced the Saints to get one first down to close out the game in the final two minutes. 

When the Saints couldn’t get that first down and punted for the third time in the fourth quarter, McCarthy led the Giants to the 14-yard line on fourth-and-1 but fell one completion short. He orchestrated three scoring drives in a seven-minute span and finished 11 of 21 for 243 passing yards, setting up a quarterback controversy for next week. 

The Giants (3-2-1) lost for the first time since Week 2 but remain atop the NFC East. 

The second-half ugliness included a stretch of three straight offensive-line penalties – drawing boos from the home crowd – and a Jameis Winston interception on the edge of field-goal range, when the Giants were trying to cut into a 24-10 deficit.

Winston’s second interception set up another Saints touchdown and was his final pass of the day. McCarthy replaced Winston for the final 12 minutes of the fourth quarter.

The Saints fueled their comeback and snapped their three-game losing streak by scoring 17 points and running 17 offensive plays in between true offensive snaps for the Giants.

How does that happen?

Daniel Carlson kicked a 58-yard field goal – a gutsy call by head coach Kellen Moore considering Carlson already had missed from 55 – with 1:23 to go in the first half and Charlie Jones fumbled the ensuing kickoff.

And then the Giants fell one play short of a goal-line stand, allowing a 2-yard touchdown pass to a wide-open Bryce Lance as decoy Chris Olave drew a double team with one second remaining.

Suddenly, it was 10-10 after a half in which the Giants – especially a defense playing tight coverage, blitzing and flying around – felt in complete control.

The Giants kneeled out the clock and kicked off to start the third quarter.

Tyler Shough, who finished ahead of Jaxson Dart in the Offensive Rookie of the Year vote last season, led the Saints on back-to-back third-quarter touchdown drives. The first – to take a 17-10 lead – was aided by a roughing the passer penalty on Kayvon Thibodeaux and finished by vintage Alvin Kamara’s slashing 31-yard touchdown catch through the missed tackles of Jason Pinnock and Jevon Holland and past the turned-around Dru Phillips.

Playing as 1.5-point underdogs and looking to improve to 4-0 at home for the first time since 2008, the Giants opened the game with the ball on their 40-yard line but went nowhere. They preferred to do damage the hard way with a 93-yard touchdown drive on their second possession.

Tyrone Tracy Jr.’s great individual effort allowed him to turn a delayed route into a checkdown catch into a 17-yard touchdown catch. Tracy caught the pass, broke to his right, ran through a diving shoestring tackle by Pete Werner and dove head-first across the pylon for a 7-0 lead.

Dominic Zvada added a 34-yard field goal before the collapse began.

Facing the third-worst rushing defense in the NFL, the Giants’ rushing attack consisted of one 20-yard gain on a reverse to Darnell Mooney and 17 yards on 14 carries by all other ball-carriers combined over the first three quarters.

Demoting the popular Skattebo sent a locker-room message but backfired on the field because the Giants didn’t have his fearless running to fall back on as the offset to their weekly lack of explosive runs.

Najee Harris (four carries for two yards), Devin Singletary (three for five) and Tyrone Tracy Jr. (three for eight) combined for nine yards as Skattebo didn’t touch the ball until the fourth quarter.