Smalling isn't bad at CB and if he plays there, am sure he'll do a decent job but it's only when he plays at RB that he struggles. Johnson is in the squad, no Walker so imagine that Smalling/Jones will be RB cover - both whose crossing is woeful.
If Rooney has a poor WC - which is plausible - then I think he should retire from internationals. Compared to other players, I don't really feel he's going to get better with age and I'd rather he used the spare time resting and with Barkley, Sterling, Strurridge a potential front three, then Rooney just doesn't have that appeal if he doesn't do well.