Craig Dahl, an undrafted rookie safety from North Dakota State, was a long shot to make the New York Giants in 2007. His roommate that season, linebacker and long snapper Zak DeOssie, was another first-year player trying to make the team.

“My head was spinning that summer trying to learn the defense,” said DeOssie, now in his ninth year with the Giants. “But Craig sat there studying and had every defensive call down perfectly. He never looked like a rookie on the field.”

A surprise of the 2007 season, Dahl became a valued contributor in the secondary and on the Giants’ special teams. But in the final game that season, during a memorable loss to the undefeated New England Patriots, Dahl tore the anterior cruciate ligament in his left knee. His season over, Dahl was eventually cut. The Giants went on to upset the Patriots in the Super Bowl.

Although Dahl was without a job, he and DeOssie remained roommates, and in the succeeding months, the two worked together in the gym as Dahl rehabilitated his reconstructed knee. Within six months, Dahl was back on the practice field. But in a 2008 preseason game, he tore the anterior cruciate ligament in his right knee and was again released by the Giants.

“A very sad day,” DeOssie said.

Heartbroken, Dahl bade his teammates goodbye and headed back to the North Dakota State campus, where he had unfinished business that had taken on new import. He needed to complete his studies in mechanical engineering.

“It seemed like there was a very slim chance I’d be back in the NFL,” Dahl said. “At times, it seemed like maybe this pro football career wasn’t meant to be.”

Dahl earned his engineering degree, but being back in a college atmosphere did more than qualify him for a job outside athletics. For a year, he was surrounded by the enthusiasm of North Dakota State players whose dreams of making it to the NFL were unbridled and untarnished by setbacks like back-to-back knee operations. To them, Dahl was proof that anyone, even an undrafted safety from a small-time program in the vast college football world, could make it in the NFL.

“When I got there, I thought it was my job to encourage them,” Dahl said. “But they ended up picking me up.”

Dahl returned to play four seasons with the St. Louis Rams, for whom he started 40 of 61 games. He then spent two seasons with the San Francisco 49ers, starting one game. In September, with the urging of Steve Spagnuolo, the Giants’ defensive coordinator this year and in 2007, the Giants signed Dahl as a free agent. The 49ers had released Dahl days earlier.

“It was cool to come back to where it all started,” Dahl said Wednesday, standing in the locker room.

He waved his arm toward the rest of the room and added, “Although the only guys left from 2007 are DeOssie and Eli Manning.”

Dahl’s return to the Giants seemed natural given his familiarity with the defensive strategies of Spagnuolo, who also guided Dahl as the Rams’ head coach. But as this season has progressed, it has become clear that Dahl’s presence in a Giants uniform is more than just a convenient reunion.

Dahl’s role in the secondary has been amplified week to week, especially against teams with aggressive passing attacks. The Giants’ starting rookie safety, Landon Collins, has been effective against the run but has struggled in pass coverage. Dahl has not only spelled Collins and buttressed the pass defense, he has also occasionally played linebacker, which allows the Giants to diversify their formations. And Dahl, now a 30-year-old veteran, is a willing mentor to Collins.

“Dahl is very valuable, just because he sees formations, understands the game and can communicate that to everyone,” said David Merritt, the Giants’ secondary and safeties coach.

In the Giants’ 1-point loss to New England on Nov. 15, Dahl was a constant thorn in the side of the Patriots. He often shadowed New England’s Pro Bowl tight end, Rob Gronkowski, and he sacked quarterback Tom Brady. Dahl was in on four other tackles, including two on special teams. He had a second tackle behind the line of scrimmage and one pressure of Brady in the passing pocket.

“Dahl was everywhere,” Merritt said. “He’s played everything: linebacker, safety, I mean, he does it all. He’s done a great job for us.”

Merritt also coached Dahl with the Giants in 2007.

“I knew exactly what I was getting when he came back — a smart player and a headsy player,” Merritt said. “His study habits are tremendous, and he’s able to help Landon in that area.”

Merritt explained that when it came to reading opposing offenses before the snap, there were defenders who were clear about the scheme they saw and defenders who were cloudy about it. Collins, 10 games into his NFL career, is still cloudy. Because of Dahl’s experience, Merritt said, he sees the landscape with learned eyes.

“He understands the game; it’s clearer to him,” Merritt said.

To Dahl, it is a skill accrued across several seasons, time he spent accumulating information on opposing teams and coaches. Dahl estimates that he spends 48 hours a week not only studying the coming opponent’s players, but also trying to appraise the opposing offensive coordinator. That is in addition to his on-the-field practice time, when the scout team mimics the opposition tactics.

“Over time, you learn how other offensive coordinators like to attack a defense,” Dahl said. “You pick up pass route concepts they favor. I take detailed notes on things they like to do. When the game starts, it lets you anticipate, which lets you play faster.”

Last week, Dahl and DeOssie both said that 2007 felt like much more than eight years ago.

“We were just bright-eyed and bushy-tailed rookies back then,” DeOssie said. “Two guys just trying to make the team any way we could.”

DeOssie has been a Super Bowl champion twice. Dahl, who received a Super Bowl ring for the first of those Giants championship teams, has since toured the NFL from coast to coast — with a stop in North Dakota.

“I’m certainly glad I have my college degree, and I’ve always wanted to explore that world more,” Dahl said. “But having a football career that has lasted this long, I’ve definitely postponed those aspirations until later.”