Yes, Sanchez' fitness will play a big part. I don't expect him to start even if fit though. I think Martial and Rashford will occupy two of the three forward spots, the other spot will likely go to Lingard or Mata - Lingard might need some rest. With Lukaku back, I reckon he'd start on the bench alongside a GK, CB and a FB. That leaves three spots for midfield options and a forward. Assuming we go with the midfield we fielded against Cardiff, two of Pereira, Fred and Fellaini will likely fill two spots, with the other spot going to Sanchez(if ready) or Lingard/Mata if Sanchez is not ready.
I think the team will be this:
De gea
Young - Lindelof - Jones - Shaw
Herrera - Matic - Pogba
Lingard - Rashford - Martial
Subs:
Romero
CB Bailly
FB Dalot
MF Fred
MF Pereira
Sanchez
Lukaku
Obviously, Ole might decide to rotate the defence and midfield, but I think that is how the Attack and bench will look like provided Martial is fit.