Jonathan Casillas had perhaps the best view. As a linebacker on the Giants’ starting defense, he was able to see up close just how energized and game-changing the new defensive line was in the first two series of Friday night’s preseason opener.

“I can tell by playing behind those guys how good we can be,” he said with a smile.

There was plenty to grin about for the Giants, whose quarter-of-a-billion-dollar investment in their 32nd-ranked defense from a year ago showed its first signs of paying off.

It was just a preseason game — one they wound up losing to the Dolphins, 27-10 — but it was enough to whet the appetite of everyone who is hoping for an about-face from the unit. Particularly up front.

“I liked it,” Ben McAdoo said of the pressure the Giants got from their line in his first game as head coach. “I’m all for it.”

The influence of the newcomers was apparent from their first snap. The Dolphins tried to run up the middle and new defensive tackle Damon Harrison gobbled him up. On the next play, defensive end Olivier Vernon — a former Dolphin — came up the middle on a stunt and clobbered Ryan Tannehill. Next it was Owa Odighizuwa who pressured Tannehill; Odighizuwa was not part of the overhaul, but given his injuries last year as a rookie, he might as well be a new guy.

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“I saw the defensive ends flying around,” Harrison said. “This one play, I look, OV shot out of his stance like lightning and JPP [Jason Pierre-Paul] was on the other side, Hank [John athan Hankins] was in the backfield. I just saw the whole defensive line working, as well as the cover guys, to give those guys a chance to get back there.”

On the second three-and-out, Vernon again put pressure on Tannehill from behind. It was the kind of pass rush the Giants were starving for in 2015, so much so that they’re paying Vernon $85 million to help provide it. “It just felt good,” he said. “It was great, man.”

Vernon let Tannehill get away from him in the end zone after he was bumped off the trail by Pierre-Paul. It’s a sign of a potential new problem for the line.

“He got back there so fast, sometimes we’ll have to watch out for each other and not create friendly fire,” Vernon said.

While the starting defense was stellar and the starting offense drove for a touchdown on the opening possession, the rest of the game was rather lackluster. Ryan Nassib threw two interceptions and lost a fumble, the three turnovers resulting in 17 Dolphins points. Starting in place of Eli Manning, Nassib was 7-for-15 for 75 yards and was sacked four times.

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“Ryan was right in line with everyone else tonight. It was a sloppy performance,” McAdoo said. “Any time you have six balls on the ground and two interceptions, that’s not what we’re looking for.”

The starting defense, though, was exactly what he was looking for. “I thought the first-team defense took a solid step tonight coming out and playing with each other for the first time,” McAdoo said before adding the one word every coach hopes to be able to use after the first preseason game: “Encouraging.”

Notes & quotes: WR Victor Cruz (groin) did not dress for the game but jogged onto the field and stood on the sideline. He could return to practice tomorrow, McAdoo said . . . Odell Beckham Jr. was expected to play, but McAdoo held him out when lightning delayed the kickoff by 50 minutes . . . The Giants’ top two draft picks both left the game with injuries. CB Eli Apple had the outside of his left knee examined and WR Sterling Shepard had a sore groin. Neither appears serious . . . Offensive coordinator Mike Sullivan called the plays, something McAdoo had kept secret.