If you are still on workers' compensation, they cannot force you to use the FMLA.  If you have been released from workers' compensation and were still injured to the point where you could not work yet.  For example, the doctor released you to light duty but there is no light duty work, you may at that time have to use your FMLA time in order to complete your treatment.  If your 12 weeks expire and you still cannot go back to work, then you potentially could be replaced without any repurcussions.  However if you are still on worker's compensation the employer cannot not unilaterally decide to put you on FMLA.  Hope this helps answer your question.