First of all, If you truly have been diagnosed by a Dr. as having Sciatica and disc issues, I personally would have you ask your Dr. if he or she would recommend massage in the first place. I also would require you to have a note from your Dr. stating that you are cleared to have massage and sometimes the Dr. will make the therapist aware of what "disc issues" you have.
There are other conditions that can mimic Sciatic pain and right at this point your question can not be answered. Not enough information known. Also, I have not heard of any "back of the knee" massage treatment for Sciatic pain. Other than that ace bandage type of contraption that is advertised on TV that when strapped around the leg it puts pressure on the back of ones calf muscle. Who know if that works?
So... Final answer to your question, check with your Dr. and maybe a Physical Therapist.
good luck!
Hermene Anderson, LMT