De Bruyne has netted 15 goals and provided 25 assists in all competitions this season, prompting interest from Manchester City, Bayern Munich and Paris Saint-Germain.
However, Allofs is confident the 23-year-old is staying put.
"At the moment, I'd call him unsellable," he told German TV show 'Doppelpass' when asked about the Belgium international. "I am 100 percent convinced he will play for us next year.
"However, in football things can develop so rapidly, that you can't say this now."