LOS ANGELES — Giants defensive coordinator Pat Graham started watching film of the Chargers’ offense on the flight from Miami to Arizona last Sunday night.

But his film prep didn’t last long. Watching quarterback Justin Herbert was too discouraging.

“I think the third play I saw Herbert drop back and throw a ball 25 yards down [the field] on a rope, and I said, ‘I’ll get to this tomorrow,” Graham laughed on Thursday, demonstrating how he closed his laptop.

Sunday at SoFi Stadium, the Giants (4-8) will be up against it trying to stop Herbert and the Chargers (7-5). But it might be even harder for Giants brass simply to watch Herbert and longingly wonder what might have been.

GM Dave Gettleman, assistant GM Kevin Abrams and scout Jeremy Breit memorably were photographed on Utah’s field at a November 2018 Oregon game while scouting the big-armed Ducks QB.

It was no secret that the Giants had an eye for Herbert, but he opted to stay in school an extra season rather than declaring for the 2019 NFL Draft.

So the Giants drafted Daniel Jones with the No. 6 overall pick that spring, and the Chargers landed Herbert with the No. 6 pick in 2020, two slots after the Giants picked left tackle Andrew Thomas.

Jones hasn’t had a great supporting cast with the Giants, but a direct comparison of Jones’ and Herbert’s production since Herbert entered the league in the fall of 2020 is staggering.

In 27 games, Herbert has thrown for 7,883 yards, 58 touchdowns and 21 interceptions with a 66.6 completion percentage. He also has 483 rushing yards, seven rushing TDs and six lost fumbles.

“Justin Herbert has one of the strongest arms I’ve seen on tape,” Giants safety Logan Ryan said Thursday. “His arm is ridiculous.”

Graham said Herbert has the size and height of the Steelers’ Ben Roethlisberger and the arm of the Chiefs’ Patrick Mahomes.

“Those guys that can just throw the ball a mile,” Graham said.

In 25 games in that same span, meanwhile, the Giants’ Jones has thrown for 5,371 yards, 21 touchdowns and 17 interceptions with a 63.2 completion percentage. He also has 721 rushing yards, three rushing TDs and 12 lost fumbles.

Most glaringly, Herbert has scored 65 TDs to Jones’ 24 in the past two seasons while playing in just two more games. Herbert is averaging 2.4 touchdowns per game to Jones’ 0.96.

“Justin is thriving as a quarterback,” head coach Brandon Staley said Wednesday.

The quality of each supporting cast matters, obviously. Herbert’s left tackle is rookie Rashawn Slater, the No. 13 overall pick that the Giants passed on by trading back from No. 11 in April.

Sunday’s playing field might be evened a bit with the absence of Chargers wideout Keenan Allen (COVID-19 reserve list). The only problem is Jones (neck strain) and Kadarius Toney (oblique) aren’t dressing for the Giants, either.

Mike Glennon, with a 6-22 career record, is starting for a second straight game after clearing the concussion protocol on Friday.

What this means is it’s now or never for Saquon Barkley to snap out of his funk and put the Giants’ offense on his back against the NFL’s second-worst rush defense (141.2).

Barkley, speaking Friday, seemingly slipped and nearly said he had a “high” ankle sprain before catching himself. His injury was previously believed to be a low sprain. A high sprain would explain why the injury cost him four games earlier this season.

Barkley said “I’m not going to make excuses,” however, for “why I haven’t been performing at a high level.” He said his ankle is “nagging,” he walked with a limp, and he wrapped it heavily in two practices this week to protect it.

But he said his bad ankle is not “the reason” for his lack of production: a season-high of 57 rush yards in Week 2 at Washington, one game of 100-plus scrimmage yards, and no touchdowns since Week 4 in New Orleans.

“I would love to be doing a lot more for my team, meaning being more productive, and you’ve got to take accountability,” he said. “You’ve got to look at yourself in the mirror and go back, watch the film and see what you can do better. These last couple games [I] haven’t been playing as well as I would like to play, especially last week with a couple drops. You can’t let that slow you down.”

Barkley said his two drops in last Sunday’s loss to the Dolphins stood out to him most in his film review. They were big plays in the game. Joe Judge and Freddie Kitchens had drawn up a game plan with Barkley as the focal point, and he let the team down.

“I had two drops, two balls that hit my hand and I didn’t come up with the play there,” Barkley said. “I felt like as the game went on, I got better in the run game. I’ve just got to continue to take what’s there. Whatever the defense gives me or whatever opportunity I have, take what’s there and when big plays are there to be made, like a catch in the flat or an under against a linebacker, I’ve got to make those plays.”

Barkley did say he thinks “the lack of time I’ve spent on the football field” due to injuries, including last season’s ACL tear, has set him back.

“Obviously, I see stuff where I’m like, ‘Damn, like I’ve got to be better there,’” he said.

But he also said his spinning pirouette on one run against the Dolphins, though it didn’t fool the defender in front of him, was important to him because “I left my feet for the first time in a minute.”

“Stuff like that, leaving my feet, it shows I know I have confidence,” he said. “It’s different to have confidence in yourself and then go out there and your body is showing that it has confidence in itself, too.”

Judge’s players are still playing hard for him, which is important to see absent consistent victories — although the Giants are 3-3 in their last six.

Ryan, a defensive captain, said “accountability,” body language and work ethic are not issues with this team. Execution is the missing piece.

“It’s a player’s league and the locker rooms dictate a lot of things in this league,” Ryan, a captain, said. “We just don’t have a locker room that’s given up. We don’t have a locker room who’s splintered. We don’t have a locker room that’s saying, ‘Man, what’s the point?’ We’ve got a locker room that’s fighting and scratching and clawing. Obviously you want the results on Sundays and that’s what we’re judged on in this league.”

The uncertainty surrounding Jones’ neck injury, obviously, puts a damper on everything at the moment. Jones could end up not playing again this season, or he could be cleared as soon as next week’s home game against the Dallas Cowboys.

The Giants just don’t know.

In the meantime, Judge and his players are gunning for the best-case scenario in the short term without him after a productive week in Arizona: a statement road victory in the City of Angels.