In January playing hockey I tore my ACL and MCL. Had surgery on February 11th was riding 8 weeks later. Of course, the PT I was doing twice a week helped immensely. I was told an exercise bike during the 8 weeks I couldn't ride would be the best thing for it and I was on it pretty soon after surgery, not a week later but it was pretty quick. I avoided the narcotics as they messed with me too much and ditched the crutches pretty quickly. It was tough but 5 1/2 months later (PT entire time) I was back to playing hockey. Its still not 100% but its close.
For now and probably a while I'm off my fixie and using a geared bike. I think I'm moving to a single speed instead