1. Lukaku is repeatedly guilty to being lazy and not making channel runs wide to alleviate pressure. A big target man central when there's no one remotely close to him is useless.
2. Because Lukaku doesn't do this it doesn't allow any space for Martial to roam central (think what Benzema does for Ronaldo, for eg.)
3. Overlapping runs don't exist. Players either don't have tactical intelligence to read the game and the run, or, as I suspect, Mourinho de-emphasises overlapping runs out of precaution.