There is no first XI. But this should be the strategy:
Spine of the team - should play in all important games, and be rested in cups:
De Gea, Maguire, Fred, Bruno, Rashford
1 Tier players - should be added to the spine:
Lindelof, Wan Bissaka, Telles, McTominay, Martial and Greenwood
2nd Tier players - substitute spine (should be used whenever we want to rest key players) :
Henderson, Shaw (Telles), Tuanzebe (Lindelof) , Matic (McTominay), Pogba (Bruno), Mata (Greenwood), van de Beek (?)
Question marks:
James, Williams
This is how you pick starting XI, based on opposition and calendar.