I think he means that on top of the occasional game against lesser teams where mata and hazard might be rested, lukaku can come on to make an impact and kind of play alongside rooney (just in front of him), if we want to come out in the second half more attacking or whatever. Unlike the situation now, where if we want to put another striker on, it has to be a like for like swap or we actually have to play 4-4-2. With rooney, we could play similarly but slot rooney alongside mata, and let him make runs of the striker. But yeah, of course primarily it is to fill the void we have at striker.