I think he could work as a squad option at a top club, not sure he's quite good enough yet to lead things week in week out. Different strengths as Haller has much better control with his back to goal but Lukaku's gone from being one of most promising young strikers in world football with a very strong premier league scoring record to a nervous wreck in pretty much a year, that's what the pressure at the very top can do sometimes.
Is Rashford or even Martial playing down the middle completely scrapped now?
I'd say having Haller as squad option and then say a Cavani (if he's available of course) as lead striker for next two seasons is what Man United need to be doing. Lukaku going to Italy or PSG to balance the books.