We should be looking this summer to move Alexis to the left-side and then move Pogba to the number 10 role behind Lukaku, then sign a defensive midfielder to play alongside Matic. So really what we need is a top right-sided player, as we'd have Rashford, Alexis and Martial fighting for the left-side. Martial doesn't work well on the right, so it makes sense if he has to go.
I don't see why he can't be used as a second striker to Lukaku, but I guess he believes his own hype and wants regular playing time which to be fair he would get at some top teams around Europe. It would be a shame to lose him but his attitude isn't really the best I don't think. Would keep Rashford and Sanchez over him any day.