Jol is keen to keep Duff at the club.
He told the Fulham Chronicle: "We've still got time. Damien's not the youngest player in the world so we will do that in the next couple of months.
"He was a very important player and was always in the top three with his fitness, although he's a bit older.
"He's one of the best pros you will ever see and always give 100 per cent. If he's fit you would always like to keep him."