The "stages of grief" is not especially well supported scientifically and probably is a stretch to invoke it here. Negotiation is a natural reaction to an employee resigning because employment is a negotiated agreement.
> The "stages of grief" is not especially well supported scientifically
As a therapy aid, no. But as a general model for describing (or recognizing) the most common experiences post loss, still considered useful.
> because employment is a negotiated agreement
This does not logically follow the rest of the sentence preceding it.
The fact that negotiation takes place at the beginning of one’s employment does not have an automatic connection to the things a manager experiences when that employee resigns.