Incredibly wasteful from United. Without wish to be churlish I don't think we learn much from this aside from the fact that Sancho might actually be a player. Middlesborough aren't great but they have come to play not "sit" the way 60% of EPL sides do when they come to Old Trafford.United have had so much time ans space to be only 1-0 up is really poor. Ronaldo and Rashford are trying to out gash each other and I can't say who is winning as yet.
Again as someone has said. You're playing Middlesbrough and have all of the ball. Wait till he plays a team at the top level...