They'll need to call up a 26th man for the DH - but they already have a 25-man roster spot for De Los Santos, opened when they optioned Anderson (announced last night, effective today). That is, unless they want to use the Anderson spot today for an additional player they can use today, and then option somebody after the DH and activate De Los Santos at that point for tomorrow night's game.
Guys on the 40-man who are eligible to be recalled as "25th man" (e.g., not under the 10-day rule) and who could be useful: Walding, Lively, Milner. That's all. Everybody else on optional assignment is either under the 10-day rule, a young starting pitcher, or injured. So, lots of options for the 26th man, but not much for the other slot. My guess would be Mitch Walding for the DH, as the 25th man, one of the relievers (Curtis, Rios, Thompson) as the 26th man), and then Walding optioned again after the DH for De Los Santos.