The Republic of Ireland international retired after earning his 100th cap during this summer's European Championships but has had a troublesome career with injuries.
The 33-year-old has three goals in seven Premier League appearances already this season - more than last year's total - and having lost Mahamadou Diarra to injury whilst on international duty, Jol is relieved he does not have to worry about Duff anymore.
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The Dutchman told Fulham Chronicle: "Of course, there's always a risk of an injury when you play these games.
"I think he's got to 100 games and he decided to call it a day, so I won't say I'm happy he did that but it's certainly got a few advantages."
However, Jol insists he would have no worries over Duff's fitness even if he did continue to play for Ireland, after amassing 56 games for club and country last year, and says he could always call on him despite his advancing years.
He added: "I feel there's no difference with Damien [whether he plays for Ireland or not].
"We played a lot of games in Europe and I could always play him. When you get a bit older you normally give them a rest but I needed them all the time."