Depends on the injury. I have an older knee injury from 1998 but it didn’t take long before I was cleared for full exercise.
Many times it’s an issue of not being cleared to bear full weight on the knee or the knee is in rehabilitation. That determine whether they can handle a bike, treadmill while walking, an elliptical, etc.
I have found that many clients know what they can do and what they can’t, but be sure to test and assess.