I felt we had a good thing when we played the 3-4-1-2 with Martial and Lukaku as a strike partnership. I think it also suited Young and Valencia who are converted wingers and so would be more comfortable with the positions of a wingback.
But I think the Arsenal game spooked the manager. We were good for 20 minutes, and then were camped in our box for most of the game. De Gea made some ridiculous saves to stop them from getting back into the game.
Agree as I think that brings the best out of both Martial and Lukaku (and the fullbacks as you say, since they're our main width anyway), while also getting Sanchez/Lingard in a free attacking #10 role which suits. Plus 3 center backs for added security. The problem was that Matic-Pogba in a midfield 2 doesn't really work. Just don't think we tried that quite enough, but also not convinced in that being a long term formation.
If we had SAF in charge with this group of players, I have no doubt that he'd get a 4-4-2 sort of formation to work with them, and get 3 of the 4 (sanchez, martial, rashford and Lukaku) always on the pitch starting, then the other subbing in for any of the 3. That's the big issue IMO. Lukaku is untouchable and never gets subbed even if he is playing like trash, so neither Rashford nor Martial get any game time in what's probably their best position. Lukaku is pretty much always the lone striker, so both have to come on the wing, when neither are wingers. Then Mourinho only plays with 2 of those 4 often. It's not the signing of Sanchez that is the problem, or shouldn't be, but the way Mourinho uses them, it makes it a problem. He puts Sanchez on the left and he always drifts inwards and never holds wide (since that's his game), so we end up with no width. So instead of having a balanced side where the manager isn't worried about resting players, isn't worried about subbing people out and gives sufficient game time to the promising younger players, we are at a situation where both are unhappy because neither gets enough game time and pretty much no game time in their best positions. How many times could Mourinho have subbed off Lukaku when we were 2 up against some shit team, brought on Rashford as the striker or Martial as the striker and given random players game time to really develop, and instead just doesn't change anything (and goes defensive, to give us headaches in the games themselves).