How about Lukaku just gets more hungry from that and starts banging in goals at the same rate as Messi/Ronaldo to rub it in his haters's faces even more? That would be the most professional thing to do.
He hates the oppo's fans for racism against him? Become a beast striker that strikes fear into them everything Inter visits them.
He hates the Manchester United fans for not standing up for him? Become an elite striker that makes the CAF have hundreds of threads saying we should not have sold him.