So I watched the first goal against Arsenal, at the beginning Valencia is pressing his player, Alexis Sanchez, he do not commit himself to much which is good, and Smalling covers him.
Since Smalling covers him and Arsenal are still on the ball, Valencia takes Smalling's place, and it is Smalling(and Di Maria) who ignores Monreal not Valencia.
Since Smalling covers him and Arsenal are still on the ball, Valencia takes Smalling's place, and it is Smalling(and Di Maria) who ignores Monreal not Valencia.