Your doctor might or might not be able to determine which of these factors is causing the knee pain. Either way, you'll probably be referred to PT. The therapist will put you through a series of exercises to identity the likely source(s) of pain in your knee. It could very well be that PN has weakened the muscles in the lower leg to the point where they can't support the knee properly. Alternatively, those tears you mentioned seem like good reasons your knee no longer functions painlessly.

How severe are those tears? I have no experience with such, so I can't offer any useful advice about them.