hi there,
first off i would like to mention that i'm not an oncologist - so take this advice with a grain of salt and run it past your doctor, or better yet is there a physio working in the hospital that you could see?
- generally speaking, the ACL prevents the tibia (your shin bone) moving on your femur (your thigh bone).
- once it's torn, it's torn really - so you won't be able to get that kind of internal stability back by exercising.
- what you could do, if you dr thinks it's ok, is strengthen up the muscles around your knee so that they will a) give you some external knee stability and b) be in great shape for when you do have your surgery so you rehab quicker. a physio could steer you in the right direction.
- the difficulty with the beautiful game is it requires a lot of stability in your knee - you need to be able to change direction fast, jump, twist - and a torn ACL will make these things difficult.

i hope that gives you a few ideas.
best of luck,
cady