Does speaking, in his absence, about the flaws of a manager who is not religiously committed, and who idolizes his superior and describes him as great, constitute backbiting (gheebah)?

Speaking about people's flaws is forbidden backbiting, unless there is a legitimate شرعية interest that cannot be achieved except through it, such as the complaint of the oppressed, or seeking help to deter evil. As for merely talking about their flaws without offering advice or forbidding an evil, it is not permissible.