Here's the depth chart with a 4-2-3-1 in mind.
Goal keeper:
De Gea, Henderson, Romero, Grant, Pereira (on loan), Kovar (on loan)
Right back
Wan Bissaka, Dalot, Williams, Fosu Mensah
Center back
Maguire, Lindelöf, Bailly, Smalling, Tuanzebe, Mengi, Jones, Rojo
Left back
Shaw
Center Midfield
Pogba, Matic, McTominay, Fred, Garner (loan)
Attacking midfield
Bruno, Mata, Pereira
Wide forward
Rashford, James, Greenwood, Lingard
Center forward
Martial, Ighalo (Until January)
That's 8 outfield players + 1 goal keeper who need to be sold in a market where no one's spending much before any additions can be made. The weakest positions appear to be center forward, left back, wide forward and attacking midfield. It would be disastrous to start the season without filling those holes.