MDS isnt an attacking fullback, he is just a fullback. Not being good enough defensively as CBs (or in general) doesnt make you so attacking (he gets forward more then a Barza or Höwedes, means little, except they are less useful in 4-3-3 then him indeed) that you need hand holding from winger infront of you, adds little offensively to warrant this special needs adapting (if its true) . Even if Cuadrado tracking back is super important for the shape Allegri likes to keep, it shouldnt be overdone IMO in terms seeing it as no choice to change if he isnt available. You bought MDS, do you trust him to play him without comprising the whole set up the team? No, then why get him. More importantly, why put him in CL list over Licht who is seasoned enough to not need this whole makeshift tactical babysitting, even if he isnt good enough starter either.
I get the pragmatic need to crowd the midfield with 4-3-3, but not only dont I think it suits Dybala to play like that (unless its CF), but we shouldnt really be too hang up on wanting to sacrifice our shape based on opponents, especially at home.