I don't think his pace is important, how many times do you see him at full pelt running against a forward. He's really great at reading the game. Also when watching his partnership with Martinez, it's obvious when Martinez charges the way he steps back to cover. He is also a good tackler, so when he has to defend against one on one he can win the ball. He really is like a rolls royce of a defender, so classy and calm and everything seems so easy, certainly not a ferrari type that uses his pace to bail him out. I do think Lindelof might be a good substitute, he is nowhere as near as good, but he'll be naturally covering for Martinez and he also rarely gets beaten (mostly because he refuses to tackle and rather trying to steer the attacker away into less dangerous areas).
Also I think what gets underrated is also how much Varane helps Dalot defensively, by giving him instructions. If you watch the games closely you can see Varane instructing Dalot when to tuck in, when to press etc. We'll se how it will work with Lindelof.
Such a shame, he is a great defender, but United should be able handle next 4 PL games without him.