ORCHARD PARK, N.Y. — The Giants have spent most of this summer soaking up the rays of their own optimism, talking about how different this year will be and how intent they are on returning to the playoffs, and practically clearing space in the trophy case for a fifth Lombardi. It’s a vibe that has been hovering around the team virtually every day since training camp began.

Every day but two.

Aug. 12, the date of the loss to the Dolphins in the preseason opener, was the first.

Saturday was the second.

The Giants lost to the Bills, 21-0, in what was supposed to be the heralded debut of Eli Manning and Odell Beckham Jr. It turned out to be a worrisome display in which the Giants went three-and-out on three of their four possessions with the starters in place and managed only 23 yards while playing into the second quarter.

“We certainly have some room to improve from this one,” Manning said.

It wasn’t all about the starting quarterback. Manning completed 4 of 9 passes for 44 yards, including a 22-yard pass to Beckham. Penalties and missed blocks that hampered the running game kept the offense from ever looking like a cohesive unit halfway through its preseason slate. Bobby Hart, starting at guard in place of Justin Pugh (shoulder), and Ereck Flowers struggled on the left side and the tight ends missed several blocks that allowed Buffalo’s defense to look impenetrable.

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The Giants had 166 yards of offense in the entire game, 67 of them on a fourth-quarter run by Bobby Rainey.

“The first-string offense wasn’t very good, to put it politely,” Ben McAdoo said. Asked if there were any positives to take out of the offensive performance, he simply said: “No.”

And yet . . .

“There’s nothing to panic about,” running back Rashad Jennings said. “It’s the second preseason game. We made some mistakes. We’re not happy . . . But we know what’s in front of us.”

Immediately, it’s the all-important third preseason game next weekend against the Jets. If McAdoo treats the preseason games as most head coaches do, it will be the last real opportunity for the Giants to flash some positives before they head into the regular season.

“Knowing how traditionally the third game is the dress rehearsal for the season, you want to go out there and put in good work, quality work, and perform well to give yourself the right to have that encouragement going into the fourth preseason game and the regular season,” Jennings said. “You have to earn that right. Otherwise, you’re just blowing smoke.”

The best play of the game for the Giants’ starters on either side of the ball came in the first quarter when safety Landon Collins hit fullback Jerome Felton at the goal line on a third-and-goal pass from the 1, forcing a fumble that was recovered in the end zone by Damon Harrison.

“When it comes down to it, the defense has to make plays,” defensive end Olivier Vernon said. “You have to . . . make something happen, especially in a crucial area like that, to be able to get the ball back for our team.”

That kept the game scoreless for the time being. It didn’t stay that way long, though.

Tyrod Taylor hit LeSean McCoy with a 13-yard touchdown pass early in the second quarter for a 7-0 lead. Collins bit on a pump by Taylor, which left McCoy open in the end zone.

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Giants running back Andre Williams fumbled the ball to the Bills, who drove for a 1-yard touchdown run by Jonathan Williams to make it 13-0 (the extra-point kick hit the right upright).

Backup quarterback E.J. Manuel found Greg Little behind cornerback Donte Deayon with 24 seconds left in the first half, then hit Dezmin Lewis for a two-point conversion to make it 21-0 at the break. That touchdown came immediately after Giants backup quarterback Ryan Nassib was sacked and fumbled the ball to the Bills at the Giants’ 19.

“We worked too hard to put a performance like that on film,” McAdoo said. “We need to take a long look in the mirror and bounce back this week . . . I don’t think I need to say much to the players. They have a lot of pride and I believe they’ll bounce back.”