MOUNTAINWALKER ~ NO, this won't derail you from your progress -- you can still lose despite all this if you keep to your eating plan. I know what I speak of -- I also had/have a badly injured right knee (which is in the process of a long, slow healing). Twice, my other leg was injured too (broken calf & torn inside knee). I am still losing even with walking with two canes, so it can be done.
Yes, I have had some months that I just maintained, but that's OK too. That's good practice anyways. So just wanted to encourage you that all is not lost. Make your #1 goal to maintain and goal #2 to continue on plan as usual. You can do this ...