I think it has something to do with Giroud being a target man, he's good when we launch diagonals into him in the box, but when he's asked to come deep to receive the ball he's slow as f***. The false 9 is (apparently) supposed to drop deep to receive the ball, but it needs a player with high mobility, which Hazard has, but maybe not the tactical discipline to work the opponent's defensive line (it isn't his natural position, IMHO). Or something like that. Hopefully that's where Higuain comes into the equation. Giroud suited France because they played with a target man up front, with Griezmann just behind him.