Imagine that you are the driver of a chariot harnessed to five strong horses. What do you need to drive that chariot to your chosen destination? You need control over the horses. If the horses are unruly—if they stand on their hind legs, try to run in different directions, or refuse to obey your instructions—you’ll never reach your destination. What’s more, you run the risk of a serious accident, which could destroy or damage both the chariot and your own body.
An injury can put a damper on your asana practice, but don’t let it keep you from practicing altogether. Yoga has so many poses that it’s usually easy to find asanas that don’t affect the injured area. Some will even help heal it.
Want to make your asana session more enjoyable and more beneficial? Then relax as you practice. Whether you do yoga for exercise, to relieve stress, or both, you'll get more out of it if you relax. Don't stress about how stiff you are; don't strain to get into a pose. Just do your best and enjoy it.