I injured myself a few weeks ago (nothing too serious but needed a couple weeks off riding). I know what you are going through and I think you should probably stick to ground work with your horse until you feel you are completely ready to train properly. You would hate to mess him up just because you wanted to ride when you weren't ready.
If there is an older quiet horse you could get on bareback or with longer stirrups that wouldn't put strain on your knee you should do that and take this time to perfect/teach some ground work!
I still haven't got on my green horse after my injury because I know he is sensitive and if I do anything wrong it will do more harm than good. I am just sticking with ground work and riding my older mare that I usually put beginners on.
Good luck and I wish you a speedy recovery!