The offside rule is not always black and white, as was seen when Ajax’s first half goal against Real Madrid was ruled out for Dusan Tadic being deemed to have interfered with play.
Under section six, there are two cases that explain why a goal can be ruled out due to another player’s inference.
The first is that a player who is in an offside position directly obstructs the goalkeeper’s field of vision, is is seen in the picture, even if he doesn’t touch the ball. This is not applicable to the case in Amsterdam, as Thibaut Courtois could, at all times, see the ball.
Referee Damir Skomina’s decision then appears to have been made on the basis that Tadic’s presence prevented Courtois from being able to play the ball.
Even then, it is up to the official to make a decision and whether or not the Belgian was obstructed remains a matter of opinion, but what is a fact is that the goal did not stand.