I feel his bad form, not all of it but some part, has to do with the fact that our players have decreased the number of times they put in early diagonals, than they did before. There were times last season when almost all our attacking play went through Valencia, with either Scholes or Rooney picking him out most times and very early hence leaving him one on one with the fullback, and he had the confidence to take him on and beat him a lot of times.
Now, with RVP coming in and our play becoming a lot narrower with Rooney having a lot more options, player having some slick interchange around the box, all that has not exactly favoured Valencia's natural ability to the extent the earlier tactic did. Nowadays, many times when he gets the ball, the other team already had him well covered and gives him very less time and space to put in a cross.
Having said that, at times when he has been one on one with a lot of time and space, he has wasted the chance so it is not entirely down to the tactic, but is could definitely have an effect.