The stock is down on the “next-man-up mentality” for the Giants.

To no great surprise, it turns out that the Giants missed the three injured starters in their secondary — veteran cornerbacks Adoree’ Jackson and Fabian Moreau, and safety Xavier McKinney — against one of the most explosive passing attacks in the NFL.

Dak Prescott challenged fill-ins Rodarius Williams, Cor’Dale Flott, Nick McCloud and Jason Pinnock even after the Cowboys’ quarterback made some early mistakes. He wound up with 261 passing yards and a 91.1 quarterback rating in a 28-20 Cowboys win on Thanksgiving in Arlington, Texas.

A few days of downplaying the absences and stressing that young players were ready for bigger roles eventually gave way to one-handed catches, crucial penalties and bad tackling angles. And when the rookie Flott suffered a second-half concussion, it just added to the short-handedness.

“It’s next-man-up mentality,” Flott said during the week. “It’s the thing you are preparing for being a backup. We always know you have to be ready at some point. I expect them to challenge me. The opportunity excites me, for sure.”

For a half, it looked as if defensive coordinator Wink Martindale might be able to pull another rabbit out of his hat.

In just his seventh career game and second this season, Williams jumped a sideline route and intercepted a pass when it looked as if Prescott mistakenly thought he had a free play. After the two-minute warning, Prescott got greedy on a third-and-15 in field-goal range and was intercepted by the Giants’ lone remaining starter, Julian Love, off of a deflection from cornerback Darnay Holmes.

The Giants led 13-7 at halftime, but CeeDee Lamb and the Cowboys’ tight ends proved to be too much over the long haul.

Lamb was a handful for Holmes — the Giants’ most-experienced healthy cornerback — in the slot. Holmes twice was called for drive-extending illegal contact penalties on third-and-long plays. The second one, however, which moved the sticks on a third-and-7 and led to a touchdown, looked like a phantom whistle by an officiating crew, led by referee Scott Novak, which is known to be among the most flag-happy in the NFL.

Lamb made a one-handed catch while fighting through pass interference and nearly added two more. He was ruled out of bounds on one that the Cowboys challenged to no avail hoping for a touchdown and couldn’t corral one with the consolation prize that he was mugged by Williams to draw a flag.

Williams made a great play to get his hand in for a break-up defending a go-route against Lamb, but Prescott was never deterred. Lamb was targeted 11 times — an ominous sign for the Giants’ short-handed secondary, with games against other elite receivers Terry McLaurin, Justin Jefferson and A.J. Brown on tap in December.

“CeeDee is a great player who is in and out of his cuts real fast and has great hands,” McCloud said during the week. “Definitely going to be a challenge.”

Pinnock, who started in place of rookie Dane Belton (the original replacement for the first two games without McKinney), missed a shoestring tackle at the 2-yard line as Dalton Schultz dove in for the second of his two touchdown catches. The entire Giants’ defensive back seven was out of place on Peyton Hendershot’s jet-sweep 2-yard touchdown run out of a four tight-end formation.