I think MSPEconomist's suggestion of a pax with diability is likely correct.
If your knee is so bad (and my sympathies, I also have knee problems and it sucks) that you cannot fly outside of a bulkhead I would suggest obtaining documentation and registering with Delta for future flights.
Being moved for disabled pax is a known risk of self-assigning bulkheads, so if you become the disabled passenger from Delta's perspective you should have an easier time of things.