My honest opinion is that you don't necessarily need a new player, I think you need to change your system. Rondon is a quality striker, arguably one of the best with development, and he is more or less doing for you now what he should be (scoring just as much as he is setting up goals for others). IMO, he should be your dlf and you should be looking for a poacher or similar player to RVN. Either that or I'd move Belhanda and Cazorla further up and play them in winger positions where they can use their advantages better (that'll get you more goals alone!)