As i said in the match thread, i don't think its whether De Gea can see the ball when DCL plays it, but if he can see the ball at any point in time between him playing it and the ball going in.
When DCL plays it is only important when determining if Sig is in an offside position, which he was. The offence only then comes when he becomes "interferes" by preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent's line of vision.
I don't think he prevents De Gea from being able to play the ball even if he cannot see it for a split second because he is clearly moving away from the ball and doesn't have enough time to get back across.