The players will have two daily workouts that emphasize conditioning and training, as well as a concentration on fundamentals. But the program isn't limited to on-field training. There are also seminars aimed at helping the players adapt to Major League life both on and off the field.
Betts wasn't invited to the rookie program last winter, mainly because he wound up rocketing through the farm system and up to Fenway Park faster than anyone expected. The Red Sox typically invite the prospects they believe are closest to the big leagues to the program.
Manager John Farrell has stated that Betts could wind up being his primary leadoff hitter in 2015.
Castillo, who defected from Cuba, signed a seven-year, $72.5 million contract with the Red Sox last August and played in 10 games for the big league club at the end of the season. He is expected to start in either center or right field in 2015.
The other players who will participate in this year's rookie program are lefties Edwin Escobar, Brian Johnson and Eduardo Rodriguez, righties Heath Hembree and Zeke Spruill, infielder Sean Coyle, outfielder Henry Ramos and catcher Blake Swihart.
Johnson and Rodriguez are two of the most promising pitching prospects in the organization. Swihart has been touted by some as the club's catcher of the future.
Several key people from the organization will serve as speakers during the seminars, including Red Sox COO Sam Kennedy, general manager Ben Cherington, Farrell, hitting coach Chili Davis, pitching coach Juan Nieves and bullpen coach Dana LeVangie.
Ian Browne is a reporter for MLB.com. Read his blog, Brownie Points, and follow him on Twitter @IanMBrowne. This story was not subject to the approval of Major League Baseball or its clubs.