It depends on which competition we're talking about:
Champions League
Ronaldo Cavani
Fernandes
Pogba Fred Vdb
Shaw Maguire Varane Awb
Ddg
5 subs: Rashford, Greenwood, McTominay, Sancho, Dalot/Telles
Premier League
Rashford (Ronaldo or Cavani) Greenwood
Fernandes
Fred McTominay
Shaw Maguire Varane Awb
Henderson
Use Pogba in rotation with Rashford and McTominay
3 subs: Ronaldo or Cavani, Sancho, Vdb
FA Cup and League Cup (as well as select PL and CL games)
Elanga Greenwood Sancho
Mejbri
Matic Vdb
Telles Bailly Lindelof Dalot
Ddg
3 subs: Mata, Lingard, Jones
I'm not counting the likes of Amad or Shoretire here as it seems they will go out on loan sooner or later this season. Maybe so will Elanga and Mejbri, but right now there's less loan talk surrounding those two. Perhaps all four will go on loan eventually this season, along with a bunch of other Academy players. In which case you simply start Mata, Lingard and Jones in these games. Martial I would look to move on now because I simply don't see how he's going to get game time. Same goes for Lingard and Jones really, though if Jones leaves you need to bring in a new fifth choice CB. With a new CDM/DLP being the priority and with a question mark over who the back-up RB should be I don't see that see that happening right now.
Rotation will be key. Consider the schedule when selecting the team.
this was he line-up in the last game yesterday against Newcastle:
Ronaldo
Sancho Fernades Greenwood
Pogba Matic
Shaw Maguire Varane Awb
Ddg
so for the next game I would start:
Ronaldo Cavani
Fernandes
Pogba Vdb
Fred
Shaw Maguire Varane Awb
Ddg
try finish off the game as early as possible, in order to put on 5 subs from the likes of Greenwood, Martial, Sancho, Elanga, Lingard, Mejbri, Lindelof, Bailly, Dalot etc.
then the following game, start:
Pogba Greenwood Sancho
Fernandes
Fred Vdb
Shaw Maguire Varane Awb
Ddg
--- Ronaldo and Cavani off the bench
then start over with:
Ronaldo Cavani Greenwood
as the front three and rotate wherever needed.
You get the idea. Obviously change and modify this depending on what competition and oppoisition it is (see my post above) and also players coming back from injuries etc. Right now there are at least three or four players injured in the squad, I think.