It is not enough that the manager is well-liked or a nice person. Sure, a nice, likeable manager earns you some points with your employees. A draconian, nasty, or controlling manager takes points away from your organization. So will below market benefits and compensation. But, a manager or supervisor, who is a pro at employee retention, knows that the quality of the supervision is the key factor in employee retention.
Almost every manager can increase her ability to retain employees by developing her management skills. Teaching a manager about how to value people can be more challenging. Particularly if the manager doesn't already value people and their contributions in her mind and heart, it will be a leap for her to change her values.