When Tom Coughlin arrived to the Giants after the 2003 season, he barely acknowledged Kerry Collins when the two passed each other in the hallway of the Giants’ facility that offseason. There was no eye contact between head coach and quarterback for a simple reason: Coughlin sensed Collins was not going to be his quarterback and had no interest in building a relationship.

Sure enough, the Giants worked a draft-day trade with the Chargers to acquire Eli Manning, and a few days later, Collins was summoned into the office of general manager Ernie Accorsi. It was Accorsi who, five years earlier, harkened back to his Penn State roots and salvaged Collins from the scrap heap. Accorsi was rewarded when Collins solved the problem at the most important position on the field and, on one glorious afternoon, threw five touchdown passes in an NFC Championship Game to guide the Giants to a Super Bowl.

Accorsi was an advocate for Collins, but when he had the chance to maneuver for Manning, business superseded sentiment. The ensuing conversation made it clear Collins, just 31, had no interest in restructuring his contract and less interest in serving as a veteran mentor for Manning until the rookie was deemed ready to move him out.

“Ernie told me that he felt Eli was one of the three or four best college quarterbacks he’s seen in the last 20 years,” Collins said back then. “Someone like [John] Elway or [Dan] Marino. Someone like that.”

So that was that for Collins and the Giants. He was released, and the clock started on the Manning era. Accorsi found Kurt Warner, and the former Rams Super Bowl champion lasted nine games before he was told to take a seat. The Giants were 5-4, and their premier players — Michael Strahan, Tiki Barber, Amani Toomer and Keith Hamilton — were thinking playoffs, not baby-sitting, and were unnerved by the quarterback switcheroo. Manning lost his first six starts, the playoff fruit died on the vine, Warner moved on after the season. But Manning’s reign, now at 15 years, had begun.

In the NFL, you are loved and needed until there is a younger, more enticing, less expensive option available. It happens. Warner was a class act — essentially a mercenary, signed for one year, hoping to resurrect his career, which he did. There was respect for him but no emotional attachment.

The end game with Manning is so tortured for all concerned because of the history, the legacy, the two Lombardi trophies and for the very reason the current general manager, Dave Gettleman, calls Manning a “mensch.’’ The transliteration, “a person of integrity and honor,” does not do justice to the true Yiddish meaning of the word. There is no higher compliment than to be considered a mensch.

It is the Eli Manning way, which is why breaking up, if this is what is going down, is so hard to do. From that very first practice in the spring of 2004, when his first pass went awry and hit a tackling sled, to his somber press conference last week following an excruciating one-point loss to the Cowboys, Manning set a standard that will be difficult to match.

For a decade and a half, his time in blue was pristine, unfettered by controversy or embarrassments or Page Six gotchas. You could set your clock to him. Covering an Eli Manning event anywhere meant showing up exactly on time, because he always was. He believed in accountability, and his trademark was to always speak to the media the day after losses because he did not want to hide. You can count on one hand the times he grew irritated or impatient or curt.

Can you recall him ever calling out a receiver for running a wrong route or ripping a coach for a rotten play-call? Teammates used to call him Easy E, and this generation likes to refer to the oldest player on the team simply as “Ten,’’ his uniform number. He used to be quite a trickster, putting blue dye in the shoes of unsuspecting rookies or changing the setting on their cell phones to Chinese. If you ask him about Twitter, he might start tweeting like a bird, living blissfully disinterested in social media noise and nonsense.

He can be seen in the team cafeteria eating with all sorts — young and old, offense and defense. Each spring, when the Giants make their pick in the first round of the draft, Manning sends the rookie a welcome text. He almost basks in being un-hip, and can put on display some impressive comedic timing, when the mood suits him. He keeps personal affairs personal and his innermost feelings behind a wall that has yet to be scaled. He shed a tear the day Coughlin was fired, and his voice grew defiant when defending himself amid a memorabilia scandal.

In a league where star quarterbacks talk once a week with the media, on a specific day and no more, Manning never sought extra interviews but granted them with a professional grace and coolness. He arrived hoping to emulate Derek Jeter’s style and ability to say plenty without saying much at all. He succeeded and left many intrepid reporters shaking their heads after a Manning session that scratched the surface, but penetrated no deeper.

In 15 years and countless interviews, in large groups and private, one-on-one exchanges, he never once referred to me by name. Professional, yes. Interpersonal, no.

He arrived a boyish, fairly awkward, single southerner and now is married to his Ole Miss college girlfriend and has three daughters all under the age of 10, with a fourth child, a son, on the way. His first Super Bowl MVP came a month after his 27th birthday, his second after he turned 31.

Since then, there has been too much losing, most of it based on the talent (or lack thereof) around him, but certainly he is not blameless. Those who point to his massive career passing numbers as merely a byproduct of his longevity do not get it. In 15 years, he never, ever missed a game, a reliability that defies logic and sets him apart. He was always available for his team, and that is the mark of a Hall of Famer.

Accorsi, way back when, could not hide his exhilaration at landing Manning, but knew there was a tradeoff.

“I feel it personally,’’ he said at the time, “because I invested a lot in bringing Kerry here. But that’s football. It’s a business, and ultimately you have to make decisions that can be painful.’’

The Giants could be there now with Eli Manning. Nothing lasts forever, and he will be fine no matter how this turns out. But the Giants will never see the likes of him again.