The announcement is being held up by the unusual logistics of hiring a manager under contract with another team, the official said on the condition of anonymity because the final procedural steps had not been cleared. But the official said a multi-year deal had been agreed to and could be announced Sunday.
Farrell had one year remaining on his contract with Toronto, where he went 154-170 (.475 winning percentage) over the past two seasons. The Red Sox have agreed to send the Blue Jays compensation for letting him leave.
Red Sox spokeswoman Pam Kenn said the team had no announcement to make. Blue Jays spokesman Jay Stenhouse did not immediately respond to a voice mail seeking comment.
Comcast SportsNet New England was first to report the deal.
The Red Sox fired manager Bobby Valentine the day after finishing in last place in the American League East with a 69-93 record that was the franchise's worst since 1965.
Farrell was the Red Sox pitching coach for four seasons before Toronto hired him as manager two years ago. He was Boston's top target when Terry Francona was fired 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 him.
The Blue Jays' disappointing season gave them a fourth-place finish in the AL East. It was a step back from the team's performance in 2011, when Toronto finished 81-81 in Farrell's first season as manager.
He helped instill a more aggressive approach on the basepaths but inconsistent pitching has been an issue since he was hired. Major injuries to several starters wore down the staff in 2012 and ace Ricky Romero had an off-year.
When Boston fired Valentine reports quickly surfaced that the Red Sox were interested in seeking permission from the Blue Jays to speak to Farrell about the vacancy.
The 50-year-old native of Monmouth Beach, N.J., has a pitching background. He broke in with the Indians in 1987 and made 109 starts over eight seasons with Cleveland, California and Detroit.
He moved into the coaching ranks in 1997 with Oklahoma State University as an assistant coach and pitching/recruiting co-ordinator. He joined the Indians as director of player development in 2001.
He spent five years with Cleveland before joining the Red Sox as pitching coach in November 2006. Boston won a World Series the next year.
Farrell was named the 12th manager in Blue Jays history in October 2010.
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 here and has worked with many Boston 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!"