The Dutchman has claimed that new terms will be offered to the 34-year-old if the Irishman is able to maintain his fitness throughout the course of the campaign.
"If he can stay fit and work hard, sometimes he works too hard, he has to relax a bit because he wants to be a good trainer," talkSPORT quotes Jol as saying.
"He's a true professional. What we will do for him is give him an extension if we feel he can play for another year."
Duff comes to the end of his current contract in June.