I have to be honest Mike, this isn’t really in my wheelhouse. In almost all situations, we advise against training with any kind of knee pain whatsoever just to be on the safe side. I would recommend working with a physical therapist to determine the precise cause of your pain, and monitor your healing to determine when it is safe to train again.
As a general rule, you will likely experience less pain on lower intensity rides, so if you can do Sweet Spot without pain, but Threshold hurts, then stick to Sweet Spot. Use your pain as a guide to determine how much your body is able to handle at this time.
We have some extensive discussion regarding knee pain in this thread, as well as some workouts and best practices to alleviate the pain. I think you will find most of what you are looking for by reading through this thread:
Best of luck with your recovery, feel free to reach out if you have any further questions.