It depends on what was said. Opinions, no matter how negative, are not defamation. And true facts, even if harmful are not defamation. So the following would NOT be defamation:"John Doe was my worst employee"--that's an opinion."John Doe did not seem to have good problem-solving skills," or "John Doe was lazy--always did the minimum he had to do," or "John does was not a team player"--all opinions."I would not hire John Doe again, or recommend hiring him"--opinion.Anything like the above is not something you can sue over. Or, say that you (for example), you were disciplined for excessive absence or lateness: if your former employer disclosed that, it's not defamation, because it's true.Only a negative false factual assertion--something provably  untrue, and not based on opinion or subjective judgment--is defamation. So say that you were never written up for poor performance or placed on an employee improvement plan or probation, but the former employer falsely claimed that you were: that, because it's a false factual assertion, could be defamation.