I haven't watched Roma a lot, but from highlights and the 2-3 games I've seen, Lukaku gets relatively many short, low passes into the box with his back to the opponent / sometimes running at the goal, it's one of the more popular outcomes of their attacks and something Mourinho obviously looks for to play to his strengths. you switch Lukaku with Vlahovic and I think we'd be cursing Lukaku for the same things that Dusan lacks.