It is not legal, but it is often done.
My former employer denied that I reported my injury immediately, threatened to fire my immediate supervisor unless he signed an affidavit claiming that I didn't report it (which he later admitted after he himself was injured on the job), completed an on the job evaluation while I was out with my injury and reported that every task I did was unsatisfactory or needed improvement (despite my 3 prior years worth of outstanding evaluations) and threatened to fire or demote anyone who offered to testify on my behalf...
Get a lawyer now, get everything in writing and let the attorney handle everything. If your employer wants to play hard ball, let them know you will not back down. GOOD LUCK!