To me, our best XI can't have Sanchez as a winger. He's not really a winger. Was much better in pre season through the middle and I feel that's where he needs to play if we're going to get the best out of him. For that reason, I would play a 442/433 hybrid in the same format France did in the world cup, with Sanchez in the 'Griezmann role'.
Lukaku
Sanchez
Fred Herrera Pogba Rashford
Shaw Lindelof Bailly Valencia
De Gea
It would also allow Mourinho to play his natural game and set up to counter teams.
Back four
Two attacking full backs, creating overloads out wide.
Centre midfielders
Two holding midfielders. Someone to distribute and someone to break up play.
Reason behind Herrera is because of the energy he would give us. To me, he is our hardest working and most dynamic midfielder. Not only that, but he's great at recovering the ball. He then has the ability to play at a quick tempo.
Next to him you have Pogba, who, whilst not as dynamic as Herrera, would provide the creativity from deep, which would make for a balanced duo.
Wide midfielders
A defensive winger and an attacking winger.
In Fred, like Matuidi, you'd have someone who is more defensive than the opposite winger. Someone who can break forward, but also have the energy and tactical nous to drop back and tuck in when out of possession.
You then have Rashford who would be our outlet. He'd have more attacking freedom than Fred and wouldn't have to track back as much. This is what Sir Alex did with Ronaldo in 2008, when he used Hargreaves on the opposite wing. A defensive winger, with an attacking winger. Similarly, Deschamps did this with Matuidi and Mbappe.
Here are a few graphics below, showing Mbappe's average positon higher than Matudi's.
The reason I'd choose Rashford is because of his directness and willingness to run off the ball.
Forwards
Target man and second striker.
In Lukaku, we have a player who, as well as being able to make runs in behind, has the ability to hold the ball and bring others into play. Then you'd have Sanchez would be given a free role. Freedom to drop off and get on the ball, but also run in behind Lukaku when the space is there.
We saw how good Sanchez was with a partner. Not only in pre season, but we saw glimpses against Chelsea when we moved him up top with Lukaku.
It would be your typical big man, small man partnership. One who holds it up, whilst the other runs off it and also links play. Lingard would also be perfect for this role.
Going forward, I'd really love to see this formation and personnel. Sitting deep and compact, and hitting teams on the counter, with Sanchez, Rashford and Lukaku. Jose would love it!