Payroll flexibility is not a problem for the Boston Red sox. They can eat and DFA a contract if they have to they've done it before.
Another thing to think about: You know that if they do start Holt in left, they HAVE to send Castillo down. There's no way he sits on the bench occupying the same role they already have Young for when the Sox have to leverage their only backup infielder in the outfield as an everyday player, it would be asinine, and it would come back to bite them if Holt is tired and can't go when there's an injury (say, to Pedroia, who nobody will admit it, but we're already all on the Injury Watch in earnest with that man, no question). They WILL assign Castillo to AAA and bring up Marrero if they have so little confidence in his bat that Holt is seen as the better option in left field, because they NEED that depth and versatility far more than they need a guy they don't trust to play effectively in the field sitting around gathering dust and effectively limiting them to a 24 man roster. No sane team would put up with that indefinitely.
So assuming basic competency in the front office, Castillo is either traded, starting in left field, or assigned to AAA, very soon now.