The Blue Jays manager is set to return to Boston, where he was a pitching coach for three years, after the Red Sox got their man at the second time of asking.
MLB: Boston Red Sox to sign John Farrell as new manager from Toronto
The 50-year-old was a pitching coach at Boston from 2007-2010 before moving to the Canadian side.
Farrell, understood to have signed a three-year deal to return to Boston, had a year left on his deal with Toronto, where he went 154-170 over the past two seasons.
The Red Sox have agreed to send the Blue Jays compensation for letting Farrell leave for their AL East rivals.
He was Boston's top target when Terry Francona was let go after the team's September collapse in 2011, but the Blue Jays would not allow him to leave.
But the Red Sox found themselves looking for a manager again just one season later, after Valentine led the team to a last-place finish and a 69-93 record that was the franchise's worst since 1965. And, after Toronto went 73-89 in 2012, the Blue Jays were willing to part with Farrell.
The Red Sox also interviewed San Diego Padres special assistant Brad Ausmus, New York Yankees bench coach Tony Pena, Los Angeles Dodgers third base coach Tim Wallach and Baltimore Orioles third base coach DeMarlo Hale.
Farrell is familiar with Red Sox management from his time in Boston and has worked with many of the club's pitchers, including starters Jon Lester and Clay Buchholz.
Even before the announcement, news of the deal trickled out over Twitter, where Blue Jays reliever Casey Janssen wrote: "Want to wish our skipper the best in Boston, good luck!"
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