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Former Boston Red Sox general manager Theo Epstein is leaving his role with Major League Baseball to join Fenway Sports Group as a part owner and senior advisor, the company announced on Friday. Epstein, who was a central figure behind some of MLB's recent rules changes, will continue to serve on MLB's competition and on-field committees on an informal basis.

Fenway Sports Group owns a number of professional sports franchises and teams, including the Red Sox, Liverpool F.C. of the Premier League, the Pittsburgh Penguins of the NHL, RFK Racing, and Boston Common Golf.

Epstein, 50, rose to fame as a member of the Red Sox front office, serving as general manager from 2003-2011. He left the Red Sox to take over as president of baseball operations for the Chicago Cubs, a position he held until after the 2020 season. Epstein won three World Series titles along the way (2004 and 2007 with the Red Sox and 2016 with the Cubs), exorcizing longstanding curses in both Boston and Chicago.

"There is no question that Theo left an indelible mark on our history that represented a transformative era," Fenway Sports Group owner John Henry said in a statement. "Welcoming him as a member of our ownership group and in the role of Senior Advisor to the broader company brings with it a sense of completion."

"I am thrilled and honored to return to FSG, to join John, Linda, Tom, Mike, Sam and the ownership group, and to serve in this new role," Epstein said in the same statement. "This is truly a unique opportunity for me -- a chance to partner with people who mean a lot to me; a chance to challenge myself in new arenas; and a chance to use my experience and perspective to help others succeed and win at the highest level.

"FSG is a dynamic, groundbreaking organization with an incredible track record of accomplishment across sports. The special connection I feel toward this organization and the ownership group has stayed with me through the years. I am proud and humbled to return as a minority owner and advisor. In this role, I will not be the one making decisions; rather, I'll be the one asking questions, offering opinions, building trust, and supporting the terrific people at FSG to help us reach new heights."

Epstein will also continue to serve as an operating partner for Arctos Partners, a sports private equity platform focused on sports.