"He'll go through a full workout in Fort Myers today," said Red Sox manager John Farrell.
Castillo, who is competing with Mookie Betts to be the starting center fielder, is gradually incorporating baseball activities, though there's still no timetable for his return.
"Rusney will start to do some dry swinging today," said Farrell. "He'll increase the baseball activities which will also include shagging BP. So he's making steady progress in that way."
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.