You are talking absolute nonsense mate.
Hazard is only Valencia's man when he's working the right channel. We don't man mark wingers, he was central and he was Herrera's responsibility.
That's Hazard next to the referee and Valencia in the correct space to stop Fabregas attacking down the line.
That's Herrera by the ref inexplicably chasing the ball towards Oscar who now recieves it from Fabregas. In this situation Carrick or Blind would be keeping goals side of and tracking the runner, Hazard.
You can see here that Herrera made completely the wrong decision, running towards a player with two men covering him already, Smalling closed him down while McNair had him covered on his left. As this is barely a few seconds later Valencia is still in position at right back, there's no way he is expected to fall back into that space, it's Herrera's job to track Hazard.
Here's the video to see how fast it plays out. Valencia does try to cover for Herrera but as he was in the correct position to contain Hazard, he can't get back in time.
Long and the short of it, Herrera chased the ball rather than tracking the player. Put simply, Herrera is not a defensive midfielder and if we had Carrick or Blind fit we most likely would not have conceded that goal.
feck all to do with Valencia.