The Dutchman watched on as Jordon Mutch's long-range strike clinched the points for the newly promoted visitors, leaving Fulham in the bottom three.
Jol lamented the late lapse and admitted that his men, who had levelled through Bryan Ruiz, need to address their defensive frailties.
"You can always concede a goal and again it happened like it has two or three times before," he said.
"It's not only about mentality, it's about quality. You have to defend especially corner kicks, I think that is the easiest way.
"I know that we can do a lot better, we had all the size in the world today but we were still losing headers.
"If you concede goals late it can be a couple of things, quality and fitness. We talk about it and we work on it."
Jol's future has been cast into doubt by a poor run of form that stretches back to last season, when they finished with six defeats from their last eight matches.
Stoke City will visit Craven Cottage on Saturday as Fulham look to halt the decline.
