@EverBlue86Sigurdsson is obstructing De Gea's view as it's not just when the ball is kicked. Look at this snapshot and you can see Sigurdsson is blocking De Gea's view of the deflected ball.
If Sigurdsson isn't there then De Gea may well have seen it more clearly and reacted to save it (I will hoy a gif up to show what I mean). De Gea seems to move to his right but only after it's hit Maguire's foot as he thinks it's going that way.
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Theres the rules. Now stop whining on ffs.