"I think we're all looking forward to seeing him on the field, no doubt," manager John Farrell said before the Red Sox's 8-4 win over the Rays on Friday.
Once Castillo gets enough at-bats at the Minor League level, he is expected to play for the Red Sox at some point during September.
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.