Jon Super, File/Associated Press

Baseball had Connie Mack. College basketball had Pat Summitt. And professional soccer has Sir Alex Ferguson.

Though American sports fans may know little about him, this all-time great announced his retirement on Wednesday at the end of the season. The manager of Manchester United for 26 years, "Sir Alex" has won 38 trophies over his storied career, including 13 Premier League championships, two Champions League titles and five FA Cups. Accolades have been pouring in from around the world, from Britain's Prime Minister David Cameron tweeting about Ferguson's achievements to the New York Times calling Ferguson "the most successful manager and coach — rolled into one — that the sport has ever seen. It is difficult to think of any comparable tenure in sports, politics or business during his time at United."

As NPR noted, "there's a case to be made that Ferguson is the most famous manager in the world — of any team sport."

That leaves a monumental challenge for his yet-to-be-named successor. However strong Man United's young players may be, however balanced the make-up of the team may be, however much Ferguson may emphasize that he's leaving on top, any successor will face impossible comparisons. Whether it's David Moyes, the expected frontrunner according to some reports, or Jose Mourinho, who himself has already earned the nickname "the Special One," any Premier League championship will just be a win. It will take 13 of them before the next manager can truly live up to the predecessor.

And it's not just Ferguson's legendary status that will make his successor's job impossible, but the uniqueness of Manchester United's situation that will make it even more enormously difficult. For one, the team's chief executive, David Gill, has also said he will step down, leaving two key positions open at the top to navigate the team's future. Moreover, the stakes are particularly high. Man United is the world's second most valuable sports team—Forbes puts it behind football club Real Madrid but ahead of the New York Yankees and the Dallas Cowboys—and it's even traded on the New York Stock Exchange. The team's prospectus even warns investors that "any successor to our current manager may not be as successful as our current manager."

Finally, Ferguson isn't going away. He's remaining at the club as a member of the board and an "ambassador" of the team. This is good in that he'll be able to advise his successor and share institutional wisdom that would be a shame for the club to lose. But it always has the potential of setting up an awkward situation. No matter how much Ferguson tries to stay out of the way and allow a successor to make his own decisions and develop his own team, his opinion will hold real sway.

Good luck to Moyes, or Mourinho, or any other manager who tries to follow in Fergie's footsteps. They'll need it.

Jena McGregor is a columnist for On Leadership.

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