The Red Sox gave Castillo a seven-year, $72.5 million deal in August, which is the largest contract ever given to a Cuban defector. Castillo is signed through 2020.
Since he signed, Castillo has played in the Red Sox minor league system.
Castillo defected from Cuba last year and acquired permanent Haitian residency papers. He was declared a free agent by MLB in June.
Prior to signing with Boston, Castillo had not played organized baseball in over a year. But Castillo had been training in the northern region of the Dominican Republic before his showcase for MLB teams in July.
Castillo played for the Cuban national baseball team at the 2011 Baseball World Cup and the 2011 Pan American Games. He wasn't picked for the 2013 World Baseball Classic, partly due to a sore arm, and left Cuba shortly after, according to Cuban baseball historian Peter Bjarkman.
The Red Sox enter Tuesday in last place in the American League East at 66-84. They are 24.0 games back of the AL East-leading Baltimore Orioles.
- Sarah Barshop