In today’s baseball world, there’s almost no such thing as a “franchise player,” at least if your definition of that term involves someone who spends the entirety of a lengthy career in the same uniform.

When Paul Olden announces the New York Yankees to the capacity crowd at Yankee Stadium on April 1, there will be two of those franchise players wearing pinstripes – and it will be the beginning of the end for the elder of the pair.

Mariano Rivera announced his pending retirement on Saturday, telling the world that his nineteenth season in the Majors will be his last. He’ll be leaving the game with an all-time best 608-plus career saves, but he’ll also leave it with a pair of longevity records; Rivera, Derek Jeter, and Jorge Posada were the first trio of teammates in major pro sports history to play together for 16 seasons, and when The Captain and The Sandman both officially set foot on the field in 2013, they’ll tie Detroit’s dynamic middle infield duo of Lou Whitaker and Alan Trammell for longevity as a pair of teammates, each twosome racking up an MLB-record 19 years in the same clubhouse together.

Jon Lane posited on Friday that “whether you’re a Yankees fan or not, saturate your mind with every one of those memories and cherish them forever” because “the Great Rivera is untouchable and you won’t see one of his kind ever again,” and he’s right – in more ways than one.

Whether you want to pin it on shorter careers, tougher economics, or any number of factors, there may in fact never be another like Rivera, or a pair like Mo and Derek: a player or players who define a generation to one team’s fans.

Personally, the entirety of the “Core Four,” but especially Rivera and Jeter, are that definition; I will turn 33 years old later this year, and for myself or anyone in my general age bracket, it was more than half our lives ago that The Sandman and The Captain came to be…and yet, to many, it probably feels like yesterday.

It was the day after Memorial Day 1995, May 23 to be exact, when Mariano Rivera debuted, starting and allowing five runs in 3 1/3 innings in a 10-0 Yankees loss to the then-California Angels. Less than a week later, the Yankees called up a babyfaced Derek Jeter, who was an underwhelming 0-for-5 in an extra-inning loss to the Seattle Mariners.

Over the years, of course, Rivera went from starter to setup man to closer to the Greatest of All Time, while Jeter went from babyfaced rookie to Yankees Captain to first ballot hall of famer…but when they started, it was nearly 18 full calendar years, five Presidential elections, and almost the entire lifetime of the U.S. Olympic Gymnastics team that won gold in London ago.

Think about that – for nearly two decades now, it’s been those two (along with Jorge Posada and Andy Pettitte, for most of the time) who have defined pinstripes, and so when one or more of those players finally decide to walk away, it’s nearly a lifetime of someone’s fandom going with them.

Children who began primary school in the fall of 1995 are now college graduates or close to it, most of those born the year the Core Four showed up are in their final months of high school and/or childhood, and the one team that has ever gotten to Rivera in the World Series, the Arizona Diamondbacks, was barely in its infancy as a franchise when The Sandman first donned pinstripes.

A lot has changed over the last two decades, a lot except for the name of the New York Yankees’ closer. But time goes on, and eventually the verb in the phrase “so-and-so is greatest of all-time” goes from “is” to “was.” Saturday morning, Mariano Rivera told everyone that his verb will change less than eight months from now, and when Paul Olden announces those same Yankees on Opening Day 2014, the Core Four will be down to the Core Two or even the Last Man Standing.

The end always comes eventually, but as Rivera has said, his wasn’t meant to be on the warning track in Kauffman Stadium in Kansas City. Instead, his end will come either in Houston on Sept. 30, or somewhere else (perhaps even Yankee Stadium) in October, or, if all goes really well, on the steps of City Hall somewhere around Halloween.

Either way, it’s coming, so Yankees fans should indeed cherish every time they hear “Enter Sandman” blare throughout Yankee Stadium – because come next winter, The Sandman will be off to Never Never Land for good.

And on that day, baseball, if not life, will never be the same again for many.