If they want to add someone to the roster who isn't currently on the 40-man, but who was (as the rules require) in the organization at midseason, they need a 40-man spot.
I'll admit that in looking at the players who fit those criteria, I don't see a lot of obvious candidates for a call up. Muñoz, Cordero, Bazardo, Seabold, and Lopez are all on the 40-man already. (I'll assume they wouldn't call up Casas to DH, although that would certainly be must-see TV.)
Utilityman Ryan Fitzgerald seems like the only guy I would consider for the ALDS who isn't yet on the 40. He would make sense if you were trying to cover a lot of positions with a single emergency backup, maybe because you want to put an injured JD Martínez on the roster but you're not 100% certain he'll be ready to play. Fitzgerald played all over the field and hit .270/.350/.512 between Portland and Worcester. It wouldn't be crazy if the FO decided that as of today, he's a better player than Araúz or even Duran, and he can cover all the positions they can in one roster spot.
Do I think that's especially likely? I do not.