Suing a supervisor for libel or defamation of character while he/she is acting as an agent for the employer is generally difficult. Depending on what he/she wrote on your evaluation, it may not be something worthwhile to pursue, especially if it was purely work related. However, if it consisted of occurrences that transpired out of the work environment, you would have a better shot at this. Since I do not know the specifics of the case, a consultation with an attorney would be your best bet to know for sure.