There's a timidity to Mata, Matic, Pereira and Martial's game, which means our structure suffers when they're all on the pitch at the same time. I'd have Martial in my first XI for his other qualities, but he needs to be joined by direct players in order for us to attack effectively.
Rashford, James, Lingard, McTomminay, Sanchez, and Greenwood all have the requisite directness to go for the jugular every time they receive the ball. Basically, at least 4 players from that list need to be playing in order for us to function near our best.