According to MLB.com, the deal is for $72 million over six years. A separate report has the deal worth $72.5 million over seven years with Castillo to join the club as early as this season.
Baseball America reported last December that Castillo defected from Cuba and was looking for a country of residency so he could sign with a major league team. He since established residency in Haiti and was cleared by the U.S. government to sign with a major league team last month.
According to Baseball America's analysis of Castillo, the 27-year-old is a 5- foot-9, 185-pounder whose main tool is speed. Others compare him to an Andrew McCutchen-type, bringing above average speed, power and average. He is not considered to be a straight slugger like countrymen Yoenis Cespedes and Jose Abreu.
During the 2010-11 Serie Nacional season, Castillo led the league with 29 stolen bases and hit .324 with 18 home runs. He would now share the outfield with Cespedes in Boston.
It was reported that the Detroit Tigers were also in the running for Castillo.