It could have been worse.
In spring training 1996, the men the Yankees were counting on to form the middle infield -- shortstop Tony Fernandez and second baseman Pat Kelly -- got hurt. They had Mariano Duncan to play second, but at shortstop... There was this exciting kid named Derek Jeter, but they weren't sure if he was ready.
So they approached the Seattle Mariners, asking about Felix Fermin. The M's wanted either Rivera or Bob Wickman... and general manager Bob Watson wouldn't do it. So they stuck with Jeter at short.
The M's ended up releasing Fermin anyway, and the Yankees got him. They sent him to the minors, and released him without bringing him up. Then the Chicago Cubs signed him, and released him later in the year. At 32, he never appeared in the majors again.
Clearly, Fermin was at the end of the line, so Jeter might have become the starting shortstop that season anyway.
But if they'd traded Rivera...
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Let's imagine that the Yankees had ANY other middle reliever in 1996, and ANY other closer from 1997 to 2011. Even a good one like, say, Troy Percival, or Robb Nen.
1996: The Yankees finished 4 games ahead of the Baltimore Orioles in the American League Eastern Division. Without Rivera, they could very easily have not won the Division. So they get the Wild Card, and would have ended up playing the Cleveland Indians in the Division Series. The Indians win the Pennant, and probably lose to the Braves in the World Series as they did the year before.
1997: No change. The Yankees win the AL Wild Card, and lose in the Divison Series to the Indians.
1998: The Yankees don't win the East by 22 games, but they still win it. And they still beat the Texas Rangers in the ALDS. But no way do they beat the Indians in the AL Championship Series. The Indians win their first World Series in half a century by beating the San Diego Padres.
Joe Torre is fired as manager. Desperate to bring in another ex-Met hero, George Steinbrenner signs Davey Johnson. (Who, in RL, was signed by the Los Angeles Dodgers at this point.)
1999: The Yankees lose the Division to the Boston Red Sox, win the Wild Card, and lose to the Indians in the ALDS.
2000: The Yankees' end-of-season nosedive means they don't make the Playoffs at all. The Red Sox win the East, and beat the Oakland Athletics in the ALDS. The Seattle Mariners win the Wild Card, beat the Chicago White Sox as they did in RL, and win their first Pennant by beating the Red Sox. In the World Series, the Mariners benefit from the Mets' baserunning blunders and Armando Benitez's choking, and win their first World Series.
Davey Johnson is fired. Hired is Yankee coach Lee Mazzilli -- another ex-Met notable, if not "hero."
Here's where it gets tricky: With the added postseason revenue, the M's re-sign Alex Rodriguez. He never becomes a Texas Ranger.
2001: The Mariners win 117 games, a new major league record. The A's beat the Yankees in the ALDS, as whoever is their closer this season blows Game 3 and Jeter's flip play goes for nought. The M's repeat, beating the A's and the Arizona Diamondbacks.
2002: No change, the Yankees win the Division but lose the ALDS... Not to the Anaheim Angels but to the A-Rod-boosted Mariners. They beat the A's, and then the San Francisco Giants in the World Series.
Now the M's are the dynasty, and the Yankees still haven't won a Pennant since 1981 or a World Series since 1978. People are beginning to talk about the Curse of Canton. No title since Thurman Munson went down in a plane crash.
2003: The story is the same, except there's no way the Yankees get through Game 7 of the ALCS with a sub-Mariano holding the Red Sox scoreless in the 9th, the 10th and the 11th. Aaron Boone never becomes a Yankee hero... and Scott Bleeping Williamson gives up the walkoff homer to Alex Gonzalez in Game 4 of the World Series, not Jeff Bleeping Weaver, and so the Florida Marlins win the World Series, and, while the Yankees haven't won a title in 25 years now, the Curse of the Bambino still lives. As Dan Shaughnessy of the Boston Globe pointed out, the Curse wasn't, "The Red Sox can't beat the Yankees," it was, "The Red Sox can't win the World Series."
Mazzilli is fired. Willie Randolph becomes the first black manager for a New York team -- not the Mets, but the Yankees.
2004: The Sox win the East, the Yankees take the Wild Card, and the Yankees lose to the Angels in the ALDS. But... the Sox lose their ALDS to the Minnesota Twins! The Angels, 2 years later than they do in RL, win their first Pennant by beating the Sox (Dave Roberts becomes a storied Sox goat by getting caught stealing), and their first World Series by beating the St. Louis Cardinals.
2005: The Sox win the East, the Indians win the Wild Card, the Yankees miss the Playoffs completely, and the Chicago White Sox still end up as World Champions.
2006: George gets desperate, trading several prospects to Seattle for A-Rod, including Robinson Cano, Melky Cabrera and the not-yet-reached-the-majors Phil Hughes. The results don't change: The Yankees win the East, but fall to the Detroit Tigers in the ALDS.
Willie Randolph is fired. Since he was fired by the Marlins, Joe Girardi is available, and George hires him.
2007: The Mariners get the Wild Card instead of the Yankees, and beat the Red Sox in the ALDS, but lose to the Indians in the ALCS. Why? Because the M's no longer have Mariano Rivera: Pat Gillick boldly sent 5 players to the M's to bring him to Philadelphia, and the World Series ends up with the Phillies beating the Indians for the title.
But having Mo and A-Rod worked out pretty well for the M's: 3 World Championships, which is 3 more Pennants than they have in RL.
2008: The final season at the old Yankee Stadium is approaching, and Hank Steinbrenner, now running the team for his ailing father, has had enough. He goes all out. He spends his father's money like there's no tomorrow, sending prospects to the Cardinals for Albert Pujols (Jason Giambi seems to be reaching the end), to the Pittsburgh Pirates for Freddy Sanchez (remember, Cano is gone), to the Marlins for Miguel Cabrera (no A-Rod, so who's already there, Ramiro Pena?!?), and to the Indians for CC Sabathia.
It seems to work: The Yankees win the East, beating out the Red Sox and the Tampa Bay Rays. They beat the White Sox in the ALDS and the Angels in the ALCS. And they triumph over the Phillies in the World Series, as Xavier Nady gets the winning hit in the bottom of the 9th of Game 7... off Mariano Rivera. The old Yankee Stadium is closed in style. The winning pitcher? The Yankees' young flamethrower from Nebraska, Joba Chamberlain.
First Pennant in 27 years. First World Championship in 30 years. Derek Jeter, Andy Pettitte and Jorge Posada finally get their rings.
2009: The first season of the new Yankee Stadium isn't as good as the last season at the old one. The Yankees lose their hunger, and lose the ALCS to the Angels, who lose the World Series to the Phillies.
2010: No change, the Yankees win the Wild Card, beat the Twins in the ALDS, and lose to the Texas Rangers in the ALCS.
2011: The Yankees lose the East to the Rays, and the Red Sox take the Wild Card.
So there it is: No Mariano, and it's 23 titles, not 27.
It can be argued that Mariano Rivera has made more of a difference for the Yankees than any player since Babe Ruth himself.
