This is exactly what you want from a keeper. If a keeper can keep clean sheets match after match and you don't notice him, it's a perfect game. Van der Sar wouldn't have had to make the saves de Gea has, yes in part because he had better defenders in front of him, but also because of his organisation and his command of the area.
And particularly, his positioning/timing when facing shots. Very rarely does a ball go right in the corner, perfectly arched away from the keeper in its flight; the amount of times van der Sar made 'good' saves with hands or feet that were more or less within his natural wingspan perhaps reduced his highlight reel (which is impressive nonetheless) somewhat, but the sheer number of these saves back then – admittedly I don't have precise figures to hand – is no coincidence. Positioning is such an under-rated/the most important aspect of goalkeeping. Everyone loves to see full-length dives saving 25-yarders but the art of being a world-class keeper is not about this.
On a related note, I'm pretty sure it's why de Gea had that phase, when he worked with Frans Hoek under van Gaal between 2014-2016, of being nigh-on impossible to score against one-on-one and often regarded as the best in the world. Obviously I wasn't there in training sessions, but I would bet you a large amount of money that positioning, along with the timing of his movements, was the foremost thing they worked on. Then Emilio Alvarez came in under Mourinho and we started to see a slight drop in de Gea's level, with quite a few balls going through his legs (and hands, on some occasions).
