#1 Mason Greenwood

Of course, number one is Mason Greenwood. He's a once in a lifetime player and he needs good development so that he's challenged and protected in the right way. His record at youth level is frankly ridiculous - he has 39 goals and 15 assists in 36 games.
We have seen what Jadon Sancho went on to produce at Dortmund and Greenwood has everything in him to do just that. He is striker by trade but one could expect him to end up getting a chance at the wide right position due to the lack of depth in the United have on the right side. Greenwood isn't just about goals, though, he's got a ridiculous pass on him and he isn't selfish in his play.
I firmly believe that Greenwood will get his debut this season but the club has to go in the right way about him. His time is going to come and he will see action and when he does, United will have a superstar on their hands.