I think this should be the lineup:
-----------Martial----------
Sanchez------------Lingard
----Pogba----Herrera------
-----------Matic------------
Shaw---Rojo--Jones--Valencia
-----------De Gea--------------
-I think the back 7 basically picks itself (Shaw over Young since I'm benching the aerial threat in Lukaku for Young's crosses anyways). --Rojo is in good form and can pass so he gets one spot, less sure about the other one.
-Lingard makes sense to play away and Pogba and Matic need Herrera (or theoretically Fellaini) in the middle to help them
-Martial has been more dangerous than Lukaku generally and is the better counterattacking player (Lukaku is good at it but Martial is better). Normally you'd consider playing the big hold up striker to ensure we don't get overrun and can counter off of punts and clearances but Lukaku struggles at that stuff and Martial would just be a constant threat to dribble past their CBs if he can isolate one. Also if Sanchez is always coming inside from LW it might open up that space for Martial to go into that vacated space, and that's where he's most dangerous.
-But Martial has been lazy defensively lately and Sanchez busts his ass and is experienced so Martial is up top and Sanchez out wide.
-Mata makes no sense away or in really in a 4-3-3 under Mourinho