WEEI.com reported that Castillo will play for Criollos de Caguas in Puerto Rico, a squad that is managed by former Red Sox infielder Alex Cora.
A baseball source said that the Red Sox are awaiting final clearance from their medical staff before putting that plan into official motion.
Castillo would get, at most, about 100 at-bats.
"We wouldn't be looking for him to play past December," Red Sox general manager Ben Cherington said earlier this week. "We want him to have some time before Spring Training, no matter what."
The Red Sox are in a unique position with Castillo, because the outfielder had a 13-month layoff from baseball before he signed a seven-year, $72.5 million contract.
Castillo did get 40 plate appearances for the Red Sox before the 2014 season ended, hitting .333 with two homers and six RBIs.
At this point in the offseason, the Red Sox have a logjam in an outfield that includes Yoenis Cespedes, Castillo, Mookie Betts, Jackie Bradley Jr., Daniel Nava, Allen Craig and Shane Victorino. There should be a spot for Castillo.
"We signed him as a center fielder," said Cherington. "Everything we've seen so far in the games he's played indicates to us that he can play that position well. As you guys know, we like to have two center-field-quality guys out there. One of them ends up in center, one ends up in right. We'll just see. We'll see how it plays out. But we know he can play center, and that's a good thing for us."
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.