sounbds like flaky table design to me
a manager is also an employee, and may also have a manager, ok there are some odd situatuations where a manager may not be employed by the company (could be a contractor or delegate from another organasiation)
the managed by should be a column in the employee table (self referncing FK to employee ID). the manager ID whould be nullable as some mangaers don't have managers (eg directors)
the alternative design is to have a separate table which identifies the entity or role
eg finance dept, accounts payable, receivable etc.... and store a manager there, and associate an employee with a department. that way round when a new manager is appointed its a single change rather than a bulk update of the employee table