This is it. Martinez obviously likes a striker to come short and link the play. This isn't Lukaku's strength. He needs to be on the shoulder. if Kone playing allows for Lukaku to clear his head of trying to be a "false 9" and focus on being on the last man, and getting in the box as often as possible for crosses than Kone will be worth his weight in gold.
Sometimes in teams you have star men and match winners and you fit other pieces of the jigsaw to get the best out of them. Lukaku is a top class striker and our other players jobs are to get the best from him. I think Kone may be the best at that.