Hey don't get it twisted. I've seen Lukaku while at Everton get the clamps put on him one on one by Danny Blind at CB.

Lukaku is useful as a flat track bully. I know his limitations but he'll be great against smaller teams which is most of what he'll face in the premier League.
Sorry but I'm not a Rashford believer.
Not all Rashford's fault because he has the tools but He lost some crucial development time with Jose. I'm off the Martial train till he shows he gives a flying one.
Martial you cannot get rid of per the board. Rashford did initially well under OGS, but looked tired (WC legs) in the last 2 months of the season, but because of injuries, he couldn't get the rest.
It's a important Year for both Martial and Rashford next season. Martial wants a perm place back in the French squad for the Euros and Rashford wants a new contract. If that isn't motivation enough for the pair of them, then get rid of next summer.
They both want to be the main striker and due to circumstance, they both got pushed out wide, you give them one more chance and let them play the #9 for the season.
United have bigger holes elsewhere behind the strikers to fill first.
Most likely new keeper?
CB, RB, 2 CM, RW, possibly cover at LB and that is before you get to replacing potentially Pogba.
That's alot needed before you even get up top.