In response to RedSoxKimmi's comment:
In response to moonslav59's comment:
I thought so too, but soxprospects has this written:
^ indicates player may be assigned outright to the minors, without the right to elect free agency, if he is designated for assignment and clears waivers
Here is my understanding of that statement, though I may well be wrong. Aceves has one option remaining, and less than 5 years of MLB service. Therefore if the Sox choose to send him to the minors, they can do so by exercising Aceves' last option, and Aceves would not have to clear waivers. Aceves would stay on the 40 man roster, and he could readily be called back up after 10 days.
Alternatively, if the Sox had no intention of calling him back up, or if they wanted to clear a spot on the 40 man roster, instead of exercising an option, they could DFA him. It's in the case of him being DFA'd that he would have to clear waivers first. He is automatically removed from the 40 man roster when DFA'd. If he does clear waivers, he would be assigned outright to the minors with no rights to opting for free agency instead.
Thanks for the explanation.