completely tore my PCL

I completely tore my PCL in January and on the road back but in that time I put some fat on and lost cardio and want to fix this. Got told not to run and to do certain workouts to build up my muscles again that I damaged along with PCL, squats and those styled workouts. Really want to get back to getting in shape though and looking to get back in the cage I have been back in gym for last month doing BJJ and MMA but my knee is noticeably weaker and less flexible. NO I am not in a rush to fight but want to work back towards that since the Doctor said I will not need surgery for this as if I build the muscles backup I will be fine. What are some good cardio workouts for a damaged knee or fat burners I can do along with diet and gym in spare time. I try to swim but its far out of way and try to do it a few times a week. I cannot run they said it is not good for my knee and that is what I have always done is just roadwork and feel lost as cardio workouts go now, anyone have any ideas? I can always just smack the bag but any other ideas would be great.

Thanks in advance
Mike