Falcao is/was possibly the best "pure" #9 in the world. The problem is, even a great #9 can be relatively ineffective without a good supply line from the midfield and wings. Van Persie is more or less as good as Falcao and what did he do in the first 3 games this season? Not much. And Falcao, as good as he is, is not Messi or Suarez, he will not create goals out of nothing. If United keep losing the midfield battle, their strikers will not see enough of the ball to be fully effective.
So I'm still not worried about United. It would have been much worse for us had Falcao joined Arsenal. Van Gaal is a great manager, but United's squad is too lopsided and of very uneven quality, he won't be able to make it work, this season at least.