The Chile attacker scored in the 5th minute, latching onto a deflection and charging into the area, before slotting the ball past Thibaut Courtois.
However, Aaron Ramsey was in an offside position when the loose ball bounced close by him in the area and the linesman raised his flag.
There was confusion briefly, before referee Anthony Taylor went over to speak with his assistant, Garry Beswick.
Taylor then blew his whistle and pointed to the centre circle, signalling that the goal would stand.
According to FIFA, simply running towards the ball doesn’t amount to being involved in ‘active play’. The laws state that a player is not committing an offence by simply being in an offside position – or even in the same area of active play for that matter.
Ramsey didn’t interfere with an opponent, he didn’t touch the ball, and he didn’t ensure his side gained an unfair advantage from his on-pitch positioning.