FOX Sports MLB insider Ken Rosenthal reports that Boston will sign Castillo to a seven-year, $72.5 million contract that is almost completely backloaded, with little money in 2014.
With this structure, the Red Sox will get a break on average salary for luxury-tax purposes while Castillo will get a record total for a Cuban free agent.
Since his workout showcase in Miami on July 26, the 27-year-old Castillo held private workouts for the Red Sox, Phillies, Yankees, Cubs, Tigers, Giants, Mariners and Reds, sources told Rosenthal.
Earlier this week, sources identified the Red Sox, Tigers and Giants as the teams still in the mix.
The Red Sox's offensive production in the outfield has been a weak point for the team this season. But this move, combined with the acquisition of Yoenis Cespedes from the A's and Allen Craig from the Cardinals appear to have gone a considerable way to addressing that weakness.
Castillo's deal will be the highest total value for a Cuban free agent. Jose Abreu, signed by the White Sox last October, still has the highest average annual value with his 6-year, $68M deal.
Castillo is represented by Roc Nation Sports, a joint venture of Roc Nation and CAA Sports. CAA's Brodie Van Wagenen and Jeff Berry handled Castillo's contract talks.