On the very first viewing Shelvey went in recklessly, went over the ball and ended up clattering into Evans.
Some times you'll be lucky and get a yellow for that, but it is the type of tackles that the authorities are clamping out of the game.
On the replays it's clear that Evans has gone in two footed. That's outlawed too, and worthy of a red. But the commentators didn't pick up on that straight away, I don't think most neutrals would have. I think most people would have only seen the terrible challenge from Shelvey. So I don't think the ref can be blamed too much for not seeing it with his one, real time viewing.
Luiz did a similar challenge to Shelvey yesterday. He was lucky in only getting a yellow. But the Stoke players and manager bitched and moaned and the after match opinion was that Luiz should have been off.