The Dutch coach saw his side succumb to a late winner that left the Cottagers in the bottom three after six games played.
"You can always concede a goal and again it happened like it has two or three times before," Jol told reporters. "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."
Fulham face Stoke City at Craven Cottage next Saturday.