As with so many legal-based inquiries, the answer to the above-posed headline question must be a qualified response, namely this: It depends.
On the one hand, if you are a worker in California or elsewhere who is on the receiving end of some unflattering statements made by a supervisor, those comments might not be legally actionable, even though they seem harmful to you.
On the other hand, though, those same comments might be grounds for an employment-based defamation lawsuit if that co-employee made them with knowledge that they were false or didn’t particularly care whether they were true or not, and the result is harm to your professional reputation.
Workplace defamation can be a bit of a slippery slope, given the elements of proof required to hold a party accountable for either written defamation (commonly termed libel) or oral defamation (slander).
Say that it has come to your knowledge that your boss passed along some less than salutary comments about you to a department head who will centrally weigh in on your request for an internal transfer. He or she did so as a matter of personal opinion, believing them to be true. That would not qualify as grounds for raising a defamation claim.
Conversely, though, if you can prove that that the comments are not true and that your boss has reasonable grounds to know they are false and uttered them regardless, you have a much more solid basis to make a legal claim. That will be especially true if the department head considering your transfer application turns down your request based upon those statements made about you.
As is readily apparent, workplace defamation can be a complex matter. An employee with any defamation-related questions or concerns might reasonably want to consult with a proven employment law attorney.