Weightloss with an injury

So I have recently undergone surgery for a torn ligament in my left knee, which i tore realy badly. this has caused some damage to my cartilage. The doctor has "recommended" I lose weight to avoid too much strain on my knee and help avoid arthritis (due to damage cartilage).
I am currently 111 kgs and would like to drop to 80 in 3 months(ish) so by the end of January.
However due to my injury I cant do cardio work (strenous) which is how i lost weight at the start of the year. (I lost 20 killos then put it all back on)
Does anyone have any recommendations or tips as to what i could do in my predicament.