Lukaku had one match where they were equals more or less. In the league games before that, Lukaku was so far down Smalling's pocket that he could just as well have played blindfolded. Smalling has also dominated Carroll, Benteke, Kane (every single time), and Vardy said Smalling is the most difficult defender to play against. Smalling even matched, or bettered, Vardy for top speed. He struggles a bit more with the likes of Aguero, which he has admitted himself as he says he is the worst attacker to play against, but he deals very well with the physically strong attackers like Lukaku and the fast ones like Vardy. Smalling had some average matched in the spring, but he has shown that he can deal with pretty much any type of attacker, so your argument is very, very flawed. Rio was a superior defender, but he often struggled a bit with the physical attackers, and Vidic, also a superior defender, struggled a lot with the fast attackers. The problem for the attackers was that there were two of them, so Vidic took out the physically imposing ones and Rio dealt with the quick ones, and very well at that. Smalling has to deal with all of them because Blind can't (he did well for the majority, but he was always going to struggle in a foot race or a physical battle with a strong player).