I'm not doctor either so take what I say with a grain of salt, but I figured since there's no joint involved, I'm not going to aggravate the injury by use and its just a matter of pain tolerance. That being said, I did tear my acl 9 years ago around the same time as a friend did. I started exercising on a stationary bike a few days post injury and again a few days post surgical reconstruction. I had a great recovery and was fully functional in 6 weeks. My friend's doctor had his knee immobilized both before his surgery and afterwards. He had a terrible result and needed a second surgery.