Rom is not a United striker, but more importantly he is not a Jose striker. Jose attacking style of play is to have a striker who can hold the play and bring others in. He did it with Drogba, reason why Lampard scored so many, again with Milito, reason why Sneijder did well, again with Costa, reason why Hazard did so well and had his first decent season.
Lukaku cannot offer that. Can score a goal here and there, but in terms of anything else, he offers very little or is not good at it. Reason why our attacks can be bland at times compared to Jose's previous teams. Lukaku just does not have the ability to keep hold of the ball and is not aggressive enough like Costa, Drogba and Milito to be a Jose type of striker.
Jose should have known this when he went for him.