Crossfit is not rehab orientated. One can 'borrow' (they like to think they own the methodology) certain aspects from them. High Intensity Interval Training (HIIT) is a widely used method to get the most out of the time you have. But, crossfit believes in overhead squats (will kill your back and shoulders), snatches (most unnatural movement ever - will definitely hurt the general guy on the street). Also they believe that you will carry on with a workout/exercise until you get the required reps/time. The main problem with crossfit is that the lactic build-up is so high that the kidneys start to battle to flush the body. I reckon you should stick to core (incl lower and upper back) and stretching for at least 12 weeks after the op. After that, rather focus on big movement exercises such as lunges, bodyweight squates, step ups, ankle hops (bouncing as high as possible from the ankles), push-ups, etc. There is always an alternative exercise to do, or a variation of one. That is why it is important to have the guidance and help that a biokeniticist (or any other that has experience in dealing with back ops) can give. Doing big movement exercises can then be incorporated into the HIIT method of for example (this is only an example!) step-ups straight in push-ups, into 1min sprint on the IDF, into ankle hops. Rest for 1min and repeat 3 times. ***Disclaimer*** I am not a medical practioner. I was a personal trainer, not anymore. Please do not construe any of the above as medical advise. I only speak from personal experience as I had a serious back injury a few years ago - broke L4 and L5. Due to proper rehab and stretching and strength training I am fully recovered.
