Stats are a wonderful thing, used to "prove" an argument like saying Nani is the better crosser because he has more assists than Valencia. For instance the Rooney winner against City took a deflection - his cross was going a lot more towards goal at the point, same with Nani's cross for Berba's overhead against Liverpool, it took a deflection. Number of assists don't tell the story but, you can at least use it to show the player is actively involved in creating goals, can't just say well the stat proves he is the best crosser.
End of the day you look at just the West Ham game and you'll see how many dangerous balls Valencia put in because he is an excellent crosser, because he didn't get an outright assist doesn't make a lot of those crosses any worse. Both him and Nani are excellent wingers that provide quality supply to our forwards. Him and Nani against Chelsea are really going to be a big difference because now Chelsea can't easily double up on Nani.