hi
whilst its true you can in some case continue playing sport with an ACL deficient knee you do need to get it strong beforehand, also working on your proprioception (balance) is very important to help prevent further injury.
a cautionary note however you do put yourself at risk of further wear and tear of the remaining ligaments and cartilage if you train/play on a knee that has more movment than is normal for you. however the stronger you get your knee preop the better and quicker your rehab progresses post op.
i agree though that the bestthing to do to get specific exs is to get a session wiht a physio as if you use the worng technique you may end up causing more damage.