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.