Man U need to get rid of Pogba asap. This guy has a bad influence on others. He's full of attitude and sideshows. Forget his passing and vision. He's not made for this club. He should be playing in Italy where the confrontational culture will put him in his place and force him to perform better. The British culture of "respect everything and anything" is not where players like Pogba can perform.
Mata, Sanchez, Martial and Fred must be sold or given away as soon as possible.
Bring in a player like Eriksen or De Bruyne or Draxler and put a defensive midfielder like Fernandinho or even Eric Dier behind them and you'll be doing much better right away.
Rashford is fine but needs a bigger figure to look up to and compete with. Lukaku is a bad influence himself so needs to be sold in exchange for a player with better work ethics. Lewadowsky, Giroud, Lacazette, Vardy and Kane come to mind.
Lingard is a good sub, but that's all for now.
Leroy Sane is a wonder player who for some reason is not liked much by Pep or even his own national team. If his head can be slapped into the right place, he's a match winner that probably wouldn't mind moving across town to Old Trafford. Seek him!
Everyone else pretty much can go, but that won't happen so start with influential changes like above and slowly get rid of everyone else.