Most of Ivan Nova's early struggles were due to a high reliance on a hitable four-seam fastball. Up until the end of the 2012 season, batters have hit Nova's four-seam for a .317 average and .495 slugging percentage according to Brooks Baseball. Nova has since added a sinker to his regular repertoire, and the results show that batters are now less likely to hit either of his fastballs. Since the beginning of 2013, the slugging percentages on his fastballs have fallen overall, and since his demotion to the minor leagues in June, Nova has allowed just a .250 batting average on the four-seam and a .245 average on the sinker. To get an idea of just how different these two pitches are, I overlayed them. There's no denying that a lot of Nova's success has been a result of the curveball, but with two fastballs in his arsenal, hitters seem to now have a more difficult task in squaring up the pitch.
Below, the four-seam is outlined blue and the sinker is outlined red.
I also created an interactive visualization back in April to show how the movement of the two pitches differ on a 3d scatter plot. Below, you can change the angle of the plot to see a bird's eye view, third base side view, and a batter's eye view of the pitch movement plotted every 10 milliseconds.