Castillo went 0-for-3 with a walk on Monday in Tampa while playing six innings in center field.
The Red Sox are fortunate to have both Portland and Pawtucket still playing, and that will allow Castillo to get the at-bats he needs to shake off the rust after not playing competitive baseball for more than a year.
"I think we've looked at it as, 'Let's get him every available at-bat before those teams shut down' before we would entertain the thought: 'Is the next step here with us?" explained manager John Farrell.
Castillo signed a seven-year, $72.5 million deal with the Red Sox on Aug. 23.
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.