Not a United-fan but they should line up something like this:
4-2-3-1
Gk: De Gea
Rb: Rafael/Jones (Rafael's injured for a while, right?)
Cb: Vidic
Cb: Ferdinand
Lb: Evra
Cdm: Carrick
Cdm: Fellaini
Ram: Nani/Valencia
Cam: Rooney
Lam: Welbeck/Young
St: van Persie
4-2-3-1
Gk: De Gea
Rb: Rafael/Jones (Rafael's injured for a while, right?)
Cb: Vidic
Cb: Ferdinand
Lb: Evra
Cdm: Carrick
Cdm: Fellaini
Ram: Nani/Valencia
Cam: Rooney
Lam: Welbeck/Young
St: van Persie