I want to address the last part of Fritz's question about how injuries effect us psychologically, regarding after-effects. Returning to the mat after injuries always effects a person- it's only a matter of how. And to clarify, by injury I don't mean a bruise or a sore shoulder; I mean an injury that severely inhibits practice and can require healing for weeks or months.
The effects can be completely irrational. A couple years ago, I sprained and fractured my right ankle in practice. I had to refrain from the mat until the doctor cleared me after several weeks. When I started again, I found myself being more cautious and taking techniques more slowly. The strangest effect was that I couldn't make myself do left forward rolls. If nage threw me, it would be fine. But in solo practice, I would just stand there for 20 minutes trying to make myself roll and I just couldn't. To analyze myself, I would imagine this is because getting injured took my self-confidence down a notch.
Every recovery is different, but I've found that following Wendy Palmer Sensei's advice works for me (albeit given in a different context). Experiment and find out where you are and what you're comfortable with doing, and also what you know you can't do...yet. Find your newly-defined comfort zone then go a little past it, but not to the panic stage. Then get comfortable with that new boundary. Then push it up a little again. And so on until you're back to where you were.
Having the little goals can make it manageable because it can take a long time to fully recover, depending on the severity of the injury. We have to be patient with ourselves. Coincidentally, I'm now recovering from a much more serious injury that has kept me from getting on the mat at all for 9 weeks now. With another ~2 months to go before normal activity, having small goals helps me. I know I can't do any semblance of ukemi but I can do slow-motion weapons kata and work on balance and footwork. It all depends on the manner of the injury, but I'd bet there's some form of practice possible no matter what the injury.
Anyway, sorry for the long post but it's a topic that's-- for obvious reasons-- a topic of interest for me right now.