Duff has been a key player for Fulham since joining from Newcastle United in 2009, racking up 94 Premier League appearances.
The former Republic of Ireland international, whose contract expires in June, turns 34 in March but Jol believes he still has a lot to offer.
Asked when he would be sorting out the winger's contract, Jol said: "We've still got time. He's not the youngest player in the world, so we will do that in the next couple of months.
"Damien is a very important player. He's always in the top three - although he's a bit older - with his fitness. He's one of the best pros you will ever see. He always gives 100%. If Damien is fit we would always like to keep him."