The young Dane faces an audition tonight in the Europa League against United.
"Christian is a clever boy. Maybe he will one day come to play for a big club like Manchester United. But I think he will only show what he is capable of here with us," said de Boer.
"It would be best for him if he was here until he is 25 years of age, because you learn a lot in Holland. If one then switches, one is a more stable player who is ready for the big leagues."