I don't think it has to do with racism, at least not to me.
I think, similar to Icardi, Lukaku polarizes opinions due to some highly visible and attention-grabbing flaws in his game.
Whether it's a tendency to react slowly, get back onside slowly, a poor first touch, or the occasional, inexplicable missed sitters (e.g. the three potentially game-winning 1v1 chances from 2m out against Barca, Roma and Fiorentina), Lukaku definitely does lack a certain sense or finesse at times.
Thankfully, in spite of the flaws, he still manages to be a fantastic goalscorer and a very useful contributor to our team.
Personally, I'm tired of the debate, but I understand why it exists - because these flaws seem so anomalous and inexplicable for someone of his repute. It's like finding out that a mathematician can't read or something. How can that be? How can an elite forward worth €80m consistently fail to trap a football? or fail to score from 2m out? How can someone his size be so easily bullied off the ball? In his most frustrating moments, I think it's safe to say that most of us have had these thoughts about him.
It seems paradoxical. Indeed, it does make you wonder what could be if he had just a tiny bit more composure and scored those easy 1v1's against Roma, Barca, Fiorentina. It makes you wonder how more effective and consistent he'd be if he improved his anticipation and close-technique. Makes you think, "surely it can't be that hard not to stand around complaining about a stray pass and just hurry back onside instead?"
I know I've had those thoughts plenty of times. At the end of the day though, it is what it is. People are complex and he is all of those things - someone who does have some rather silly weaknesses, but still manages to be one of the top footballers in the world, and a highly valuable asset when utilized correctly.