Five years after those forgotten instructional league games, Posada, Rivera and Derek Jeter would call themselves big league teammates for the first time, kicking off a record-setting run that is the longest across the four major professional sports.
When Rivera made his first appearance of the 2010 season in Tuesday's 6-4 Yankees win over the Red Sox, he joined Jeter and Posada as the first trio of teammates in MLB, NBA, NFL and NHL history to have played together in 16 consecutive seasons.
"It's hard to believe," Rivera said. "It's not because of who the players are; it's just that the right things have to happen for everybody. You've got to have the right team -- the team willing to spend the money to keep you. All of that stuff has to happen, and it did. It's amazing."
The previous Major League mark of 15 consecutive seasons was held by the Brewers trio of Jim Gantner, Paul Molitor and Robin Yount from 1978-92.
But Jeter, Posada and Rivera have held the pinstriped honors for some time. The second-longest trios in Yankees franchise history played 13 years together -- Bill Dickey, Lefty Gomez and Red Ruffing from 1930-42; and Whitey Ford, Elston Howard and Mickey Mantle from 1955-67.
Rivera saw his first Major League action in 1995, making 19 appearances (nine starts) with the Yankees. Jeter made two stints with the big club and got into 15 games that season, Posada made his big league debut in September, and the rest turned out to be history.
As they dressed for yet another encounter against the Red Sox within the cramped confines of Fenway Park's visitors' clubhouse, Jeter slung a sanitary sock over his shoulder and said that it was difficult to imagine taking the field without Posada or Rivera in uniform.
"It's almost like they've always been there," Jeter said. "It's just sort of the normal thing. I don't think you ever sit down and say, 'Well, here we are again.' It's just, you get used to playing with those guys. You get accustomed to it, I guess."
Growing from rookies to five-time World Series champions, Jeter, Posada and Rivera have taken on stronger leadership roles. Andy Pettitte also belongs in that group, though his three-year detour with the Astros disqualifies him from making the trio a quartet.
"We all have the same mentality -- to fight," Rivera said. "We're always fighting, always trying to do the job, always trying to win for the team. And winning -- that's what keeps us together. If you win today, it's over. Tomorrow is a new day, and we have to win tomorrow."
Having such recognizable key cogs on the roster produces a certain calming continuity, especially with the inevitable turnover that transpires after each season -- even the championship years.
"I think it's a good situation when you look up there and everyone is looking in, and you see the same guy almost every day," Yankees manager Joe Girardi said. "There's something constant about that. There's something that brings a normalcy to the game."
The Yankees tweaked their roster over the offseason, hoping to repeat as World Series champions in 2010, but they did so knowing that many of their key parts would not change -- especially their three longest-tenured performers.
"The attitude doesn't change -- it really doesn't," Posada said. "Even when we didn't make the playoffs, you put this uniform on, and it's different than putting another uniform on. The Yankees are really different."
With Rivera's first pitch against the Red Sox on Tuesday, the franchise officially played host to something that no trio in baseball has experienced. That included a certain catcher and shortstop who once shared a rented Dodge Neon to commute to Yankees home games from a New Jersey hotel in 1995.
"We all pretty much grew up with the same philosophy in terms of winning," Jeter said. "We came up in this organization, and that's all anyone cared about -- winning. That's how it is for us.
"The bottom line is that everyone wants to perform well, but you're trying to win. You have to have that mind-set."
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.