Unless you want to squeeze the image, you'll get black bars (letterboxing) because they are different aspect ratios (same depth but 108 pixel difference in width)
I suggest that you create a composition of 1920 x 1080 and drop the 2K material onto it. It will mean that you crop 54 pixels on each edge of the frame - but it's probably the only option if you don't want letterboxing or squeezing,