You can use substitute. In your example the new employee is a substitute for one who resigned the job.

You can use substitute. In your example the new employee is a substitute for one who resigned the job.

The Vocabulary.com describes the substitute as :

Something or someone that takes the place of another is said to be a substitute. You may be sent into a game as a substitute for an injured player.

The new employee who joins is a replacement for the former.

You can use substitute. In your example the new employee is a substitute for one who resigned the job.