expression
hard to replace (with another person); irreplaceable (person)
Fixed expression meaning that a particular person is so valuable or unique that no one else can take their place. Often used in formal or written contexts to praise someone's abilities or character.
彼は余人をもって代えがたい人材だ。
He is a talent who is hard to replace with anyone else.
このプロジェクトには、余人をもって代えがたい専門家が必要だ。
This project requires an expert who is irreplaceable.
かけがえのない is a more common and general adjective meaning 'irreplaceable' for things or people, while 余人をもって代えがたい is a formal, set phrase specifically about a person being irreplaceable.
A set phrase composed of 余人 (another person), を以て (by means of, with), and 代え難い (hard to replace). The expression is literary and formal, emphasizing the unique value of an individual.