GLENDALE, Ariz. – As the celebration died down and teammates all around him began filing out of a raucous locker room, a subdued Xavier McKinney soaked in the hard truth.

The Giants overcame a 20-point halftime deficit (and a 21-point third-quarter deficit) in a 31-28 victory Sunday against the Cardinals.

They did what they were unable to do one week earlier, when a 16-point first-quarter deficit mushroomed to 26 at halftime and 40 by the end of the game.

There’s an easy-to-identify pattern of slow starts putting the team in near-impossible spots.

“In a way, we are happy we won, but we CANNOT keep doing this,” McKinney told The Post. “In this league, the amount of good teams you play, I don’t think you can sustain the slow starts that we are having. And we have to fix that ASAP.”

It was important perspective not to be lost amid the final score. 

“If nothing else, it was just important that we fought and got back in the game,” receiver Darius Slayton said. “As a collective, we showed that we can be collective and we can win games.”

Here are some other takeaways from the Giants’ season-saving – no, that is not an exaggeration – Week 2 win:

1. Saquon Barkley’s dilemma

If an MRI exam on Monday confirms Barkley has an ankle sprain and nothing more severe, then what? Figure he misses Thursday’s game against the 49ers at the very least. 

This is the problem with having him on a one-year contract: His best interests don’t align with the team’s best interests.

Barkley certainly will have people in his ear urging him to make a “business decision” and wait until he is 100 percent healthy to retake the field because he cannot risk that a more severe long-term injury will carry into his offseason free agency.

Will he tune out that chatter – like he did when he showed up for training camp without any kind of holdout after his contract negotiations broke down and he was stuck with the franchise tag?

Barkley’s modus operandi after sprained ankles has been to rush back too soon out of team-first intentions. After missing three games in 2019, he averaged just 2.8 yards per carry in his next five games. After missing four games in 2021, he averaged 3.5 yards per carry over his next six.

It took a while in both cases for him to return to form. 

What will his absence do to his future?

In one way, it reinforces the durability concerns that were used against him in contract negotiations.

In another, it could save him some wear-and-tear off his final total of carries at the end of the season.

It also gives the Giants a look at their offense without him. Which side does that favor? Depends on how his backups and a passing-oriented focus performs.

The “don’t pay big money to a running back” crew would have a hard time saying that any late-round rookie could be plugged in and make the special play Barkley did to score his second touchdown against the Cardinals — or play 66 of 68 offensive snaps like Barkley did – unheard of for a running back in today’s game.

2. Momentum believer

Leonard Williams can’t deny it any more.

After the way the Giants fell down a slippery slope last week and one blocked field goal snowballed into a 40-0 loss to the Cowboys, Williams still wasn’t sure he believed in momentum.

But seeing the way that the Giants scored on five straight offensive possessions and that fed into three straight defensive stops and a comeback, the veteran defensive tackle is buying in.

“I leaned over at one point and talked to [defensive line coach Andre Patterson] and said, ‘It’s crazy that momentum is a real thing,’” Williams told The Post. “You hear that a lot and as a player sometimes I don’t want to believe it, like, ‘Oh, that can’t be it.’ But it’s a real thing.

“In the second half, we started getting momentum and we finished the game.”

3. Generational moment

Isaiah Hodgins can remember his father, James, playing home games for the Arizona Cardinals and introducing him to teammates like NFL legends Kurt Warner and Larry Fitzgerald.

All these years later, Isaiah caught the game-tying touchdown in the fourth quarter with his own family in the crowd at State Farm Stadium.

“My son and my parents were right there in the end zone, so I was able to give the ball to my son,” Hodgins said. “It was his birthday, so it’s definitely a birthday for him to remember. He won’t know it now, but he’ll look back on it and it will be a cool moment.’’

4. Surviving line

Left tackle Andrew Thomas expected to test his injured hamstring in pregame warm-ups but that didn’t happen. He was part of the inactive list made official 90 minutes before kickoff.

“The timing didn’t work out with being inactive or not,” Thomas said. “I just did some treatment and stuff like that just to rest it and give it time.”

Thomas still feels he is “progressing.”

“It’s tough without going full speed in the game to know,” Thomas said.

In Thomas’ place, Josh Ezeudu – a second-year guard – made his first start at left tackle since his college days. His former North Carolina teammate, right guard Marcus McKethan, made his first career start in his second career game.

“They did a heck of a job, those young guys on the offensive line,” head coach Brian Daboll said.

The pass-block win rates for right tackle Evan Neal (93.1 percent) and Ezeudu (89.7 percent) ranked No. 15 and No. 21, respectively, among tackles, per ESPN.

Mark Glowinski (93.8 percent) – who entered the game for a concussed Ben Bredeson in the third quarter – and Marcus McKethan (89.7 percent) ranked No. 18 and No. 27, respectively, among guards.

Rookie John Michael Schmitz (100 percent) tied for No. 1 among centers.

It was a major step forward after the Week 1 implosion – albeit against inferior competition to the Cowboys’ pass rush. One point to correct: Too many penalties, including a hold against Neal that briefly took a touchdown off the board. 

5. Play call me

The easiest explanation for how the Giants’ offense went from scoring zero points over the first six quarters of the season to scoring on five straight drives in the next two would be that Daboll took over the play-calling duties from offensive coordinator Mike Kafka.

Daboll pointedly denied this happened after the game.

But he wasn’t surprised by the question and seemed to even understand that it came from him holding a play sheet, which raises some eyebrows. Maybe that is just a good job by Giants media relations prepping the coach for a topic that gained steam on social media during the game.

“We always communicate like that,” Daboll said. “I always have a sheet with me. But Mike did a fantastic job. I communicate with Mike. I communicate with T-Mac (special teams coordinator Thomas McGaughey). I communicate with (defensive coordinator) Wink (Martindale).

“So, the offensive staff did a great job. Moved the ball, scored points. Did their job.”

The internet sleuths at Talkin’ Giants clipped television images of Daboll holding a play sheet in the first half that seem to validate his response. Let’s just say it will be a focus of Thursday’s game.