The problem with Rom is that while he draws attention, he doesn't pull players away to create space for others. He's incredibly static, so he's man marked but to the point where the centre-half doing it is always in position to cut out anything else that develops from an oncoming attacking mid or whatever.
Rom's position in the modern game is the most important for a club. Goals are important, yes, but the truly great strikers in the game today are ones who constantly roam, take the ball and bring others into the attacking phase. Rom does none of that often enough to be relied upon. Think about how important Suarez was to Liverpool - without that focal point, they wouldn't have got near the title that season, would have struggled to make the top six. Look at how an on form Costa transforms Chelsea, or how City look a different team entirely with Aguero.
In the modern game, I class a striker who moves intelligently and brings the whole team up the field and forward as a cohesive threat as a much more valuable asset than a poacher. Lukaku needs to add more strings to his bow to be considered a truly top drawer footballer.