The Dutchman has battled for form since his January arrival from Manchester United.
"We have to be able to use him a lot, of course, Memphis," said Génésio. "But he has not played at Manchester United for a long time so he lacks rhythm.
"He will also have to adapt to the workings of the team. It is a pity that he can't play against AZ, his replacement will not be easy to choose."
Memphis is Cup-tied from tonight's Europa League clash with AZ.