Can my employer fire me for an injury that i do no have?

If you want an answer to this question, you are going to have to provide some more detail.  Are you an at-will employee or does the jurisdiction you work for have a civil service system?  Do you have any idea why your employer thinks you have a knee injury? Does your employer think you suffered the knee injury on the job or off the job?  How long ago were you fired?  How long had you been with this employer? Do you think the real reason for the termination was a mistaken belief that you had a knee injury or do you think the real reason is something else and if so, what?