Depends on how harsh you injury was to begin with I’d say. If it was just a cut to the skin then I’d say go for it, if you re-tear it it will bloody hurt, and the hospital might be a bit annoyed with you, but I wouldn’t think there would be any long term repercussions (not that I have any medical knowledge). Best thing to do would be to give the appropriate medical practitioners a ring to find out the average healing times for your particular injury, and go from there.
If it’s OK to ride then it’s OK to race on it – as long as you cover any wound with protection and padding, I’d give it a go. Depends on the injury I guess, and also if riding aggravates it. When I’ve sought advice from various practioners, it always seems to be down to what I think, how I feel, and if there is stress on the injury.