I hope I'm wrong but I'm of the belief we will be looking for a top no. 9 once Martial let's Ollie down as the main man up top. Again, hope I'm wrong, but I can see that happening.
My hope is that in the meantime Greenwood is ready to take over the no 9 role when this transpires.
If it were me I would try to think like Fergie and plan two steps ahead, sign Kane now with Greenwood to learn from a proper no. 9 and eventually replace him. Use Martial, Rashford and Sancho for the wings. That would be perfect, a big ask, but we are a big arse club and need to behave like one.