Depends what Ole wants (if we sell Lukaku), is he looking someone who will be a starter from a get go or he is looking some bench warmer on lower wages, that way Rashford gets free path towards starting XI.
People already mentioned some names that might be able to lead a line from the start but if he is looking something different, plan B, some target man type (not like Lukaku in JM vision of that role), someone who wont make a fuss for being on the bench etc.
Well, maybe we can look at player types like Mitrovic, they aint better strikers than Lukaku but in target man role with focus on build up play( lets be real in that specific role, Mitrovic looks like Pele compared to Lukaku), play of them, being able to bully defenders (being a bit of cnut on top of it), someone who you can bring in last 10 min (at home) if you wanna try something different etc. Maybe there are cheaper and better options similar to this somewhere in Europe, dunno.