cant expect your defenders to be pushing your counter attack.
you need pace upfront and in midfield to do it. if your defender is counter attacking he is, in theory, running past the midfield to get involved.
that united goal is completely text book, three very quick players, running apart from each other, splitting the defenders. gave the defenders no chance, it just put the pressure on the guy taking the shot, all the other work is made easy buy their pace and running off the ball.