Epstein explained that it was his plan all along to leave after the 2012 season, when his contract was set to expire, and he would have served 10 years on the job.
A former sportswriter for the Yale student newspaper, Epstein put his journalism background to use in a thoughtfully worded piece.
"For the last decade, I gave everything I had to the Red Sox and received even more in return," wrote Epstein. "I grew enormously as a person, had some successes, and made a lot of mistakes, too. I still love the organization, enjoy close relationships with owners John Henry and Tom Werner -- as well as a complicated but ultimately productive and rewarding relationship with Larry Lucchino -- and count many of my co-workers among my dearest friends. The reason I am leaving has nothing to do with power, pressure, money or relationships. It has nothing to do with September, either.
"Football legend Bill Walsh used to say that coaches and executives should seek change after 10 years with the same team. The theory is that both the individual and the organization benefit from a change after so much time together. The executive gets rebirth and the energy that comes with a new challenge; the organization gets a fresh perspective, and the chance for true change that comes with new leadership. This idea resonated with me. Although I tried my best to fight it, I couldn't escape the conclusion that both the Red Sox and I would benefit from a change sometime soon."
Once the Red Sox endured a September collapse of epic proportions, ultimately resulting in a managerial change, Epstein decided it was best to move his timetable for leaving up by a year.
"Then, September happened," he wrote. "All of a sudden, we found ourselves needing to pick a new manager, a decision with long-term implications and one best made by someone who could lead the Red Sox baseball operation for the foreseeable future. Then the Cubs asked permission to interview me. The Cubs -- with their passionate fans, dedicated ownership, tradition and World Series drought -- represented the ultimate new challenge and the one team I could imagine working for after such a fulfilling Red Sox experience."
While Epstein could have been the one to select a new manager and worked with that person for a year, he said he believed it was best for the organization for him to leave now.
"So, knowing my time as the general manager was drawing to an end, I had a decision to make: stay one more year and do my best to conduct the manager's search under less than ideal circumstances, or recommend the succession plan, allow Ben [Cherington] to run the search process, and join the Cubs," Epstein wrote. "I wrestled with leaving during a time when criticism -- deserved and otherwise -- surrounded the organization. But Walsh's words kept popping into my head, and I recalled how important it was for me as a relatively new general manager to bond with Terry Francona during the interview process back in 2003."
That said, the decision to cut the cord with the Red Sox was still painstaking for Epstein, who grew up in Brookline, Mass., which isn't more than a titanic David Ortiz home run away from Fenway Park.
"It was very difficult deciding to leave the place where I grew up, where I met my wife, where my son was born, where my family and closest friends live, and where I help run a charitable foundation," he wrote. "And it was equally hard to part with the organization and the people, including John, Tom and Larry, who entrusted me with this role at such a young age and supported me along the way. But it was the right thing to do."
Epstein got sentimental at times.
"What a privilege it has been to be a part of the Red Sox these last 10 years. The first title in 2004, born from the heartbreak of Aaron Boone, was unforgettable: The Steal, Papi, the Bloody Sock, the Greatest Comeback Ever, the end of The Curse," Epstein wrote. "The second, 2007, was equally rewarding, as it solidified the franchise's rise and marked the emergence of a core drafted, developed and trained in the 'Red Sox Way' so many had worked so hard to establish."
Epstein expressed pride about the organizational philosophies that developed under his watch.
"Beyond the results on the field, I believe the Red Sox came to stand for certain things over the last decade. Pride in the uniform. Appreciation of our history. Controlling the strike zone. Grinding at-bats. Having each other's backs. Rising to the moment. Never backing down. Connection to the fans. Hard work. Playing with passion and urgency," Epstein wrote. "These concepts were taught in the Minor Leagues and reinforced at the big league level by our homegrown players and by Tito, a selfless leader who always put the Red Sox first. These principles united the organization and came to define us."
But as Epstein would say, "then September happened."
"This is why September -- when we let fans down by falling short of these ideals -- was so crushing. But the Red Sox will recover. What was built up with pride and passion over so long cannot be torn down in one bad month. The same is true, I know, of the fans' loyalty.
"Yes, September was a collective failure. As the general manager, I am the person ultimately responsible. Things did indeed happen in the clubhouse that do not have a place at the Red Sox or anywhere in sports. But the reports about team-wide apathy and indulgence are exaggerated. It may not seem this way now, but the team did care about winning, about the fans, and about each other; unfortunately, we failed when we let less important things get in the way. I tried desperately to reverse our slide, as did Tito, the coaches, and the players. But we just could not play well, and then we did not handle the adversity well."
In Cherington, who will be formally named GM on Tuesday, Epstein believe the Red Sox have the ideal successor.
"If not for the complete confidence I have in Ben to address these issues, I could not in good conscience leave the organization at this time. But there is no one in baseball more qualified to be the next general manager of the Red Sox," Epstein wrote. "Ben is infinitely more prepared than I was when I took over nine years ago. He's been an area scout, an international scout, an advance scout, a farm director, and he's supervised drafts. Ben is honest and insightful, fearless and friendly -- and he is ready to lead this organization forward."
Epstein emphasized that the team's historically poor finish, in his mind, was nothing more than a blip.
"Lost in the cacophony of the last few weeks is the fact that the Boston Red Sox remain one of the preeminent organizations in baseball, with an extremely bright future. Ben will head the same scouting and baseball operations staff that engineered much of the club's success the last nine years. There is tremendous talent on the Major League club and significant depth in the farm system after a decade of strong drafts."
He also backed the ownership of principal owner Henry, chairman Werner and president and chief executive officer Lucchino.
"Despite recent criticism, Red Sox ownership remains a model for others to follow," Epstein wrote. "John, Tom and Larry demonstrate their commitment to winning in the most fundamental way possible: If something needs to be done to help the team on the field, it gets done. September happened despite them, not because of them. It may not seem this way now, but I am convinced that we will look back at September of 2011 not as some harbinger of the demise of the Red Sox, but as an anomaly in the midst of a decades-long run of success for the franchise.
"Some good may even come from it. I know the climate is especially hostile right now, and our mistakes are well documented, but I encourage fans not to lose faith in the players or in the organization. Red Sox Nation is a fantastic place, and it's even better when we take a deep breath and give each other the benefit of the doubt."
Epstein assured readers that he will be rooting for the Red Sox from a distance, something easier to do when he runs a team in the National League.
"In Fort Myers [Fla.], next spring, a rededicated, revitalized, reborn Red Sox team will take the field for its first workout of 2012. September will seem like a long time ago. So will 2004 and 2007. I won't be there, but the 12-year-old in me will be rooting for the Red Sox [except, of course, when they play the Cubs in June]. From afar, I think I'll finally be able to enjoy the experience more, to pull for the players with appreciation for their hard work and better perspective when things don't go the way we -- or they -- want. I hope you will join me.
"Thank you for all the incredible support this last decade. I will never forget it. May we meet again in an October not too many years from now."