I own both the 85 1.8, the L, and the 70-200 2.8II.
My rather non-committal comment earlier was based on the fact you may want to invest in something new based on the way your post was phrased. The 85 f/1.8, if you can find a place for it in your line up, is a great lens.
For portraits you are not going to get the look from 70-200II at 2.8 that you will get from 85 f/1.8 at f2!!! they will look very different. The canon 85 is a dedicated portrait lens... 70-200II, while capable of taking great portraits, is a jack of many trades.
The reason Canon has not updated 85 f/1.8 since 1992 (20 years!), is not because they are lazy, but IMHO because they feel it is still good today, and has relevance.
Also, sharpness is not everything in portraiture, though 85 1.8 is plenty sharp. I will have to give the nod to the zoom on sharpness across the frame though 85 has really good center sharpness which is what you go for in portraiture for the most part.
Importantly, at the same focal length of 85mm, my passing impression was that there was a little more compression with the zoom than with the prime. I haven't tested this very carefully, but you shouldn't expect the same framing or "look" from the two lenses.
They are both competent in their own right.