If you haven't experienced some kind of an injury by now, then you're either very lucky or just haven't been training long/hard enough. Injury is something that everyone will encounter eventually, regardless of whether they train or not. And the number one reason for getting hurt is being weak, so you're a lot safer overall by hitting the gym.
The hardest thing about injury is being unable to do the things you could before. Pain-free movement is easily taken for granted until you experience the other side of the coin.
What to Do
There are many ways to rehab an injury, but what follows is just what I've found to be most effective.
Basically, you will probably see the most amount of progress by actually practicing whatever movement you want to be able to do. The key is to practice it absolutely PAIN FREE.
You must stick to this rigidly.
This means no progressing the weight if you start to get even the smallest niggle. Not only will this further aggravate the damaged area, but it will also inhibit your ability to focus on your technique and may create a subconscious pain association between your brain and that exercise.
Case Studies / Examples
I trained a client who had really bad knee pain. Just performing a bodyweight squat caused irritation. So, I started him squatting to a VERY high box where he experienced no pain. It took a while, but we gradually dropped the height of the box until he was at parallel, and pretty soon he was able to use weight with no pain whatsoever.
The second example is myself. In my entire time training, I had never been able to do a pain free dip. As an experiment, I began doing dips with a thick resistance band under my knees to take some of my body weight. I stayed with this band until I could do 20 paused reps with absolutely no pain. I gradually decreased the band thickness until I eventually progressed to the point where I'm at now: able to do extremely deep dips with effectively no shoulder issues!
I'm not a physio, and I know there there is much more nuance to some injuries. But it's my experience that a lot of rehab information is needlessly overcomplicated and often ineffective.
If you don't believe me, try just working the movement you want to rehab. The key is to be okay with reducing the intensity to whatever weight is necessary for you to do it pain free. Again, you must be very strict with yourself. This takes a great deal of patience and consistency. I promise you that if you rush the process, you'll end up back at square one or worse.
Hope that helped!
Happy training guys,