The Cottagers are still adjusting to the loss of Dempsey, who joined Tottenham last August, and Mousa Dembele, who moved to White Hart Lane in the same month, according to Jol.
"Clint Dempsey always made the difference," said the Dutchman, who was focusing on Barclays Premier League survival ahead of Saturday's clash at Norwich.
"I feel if Clint Dempsey hadn't been here over the last three or four years, we could've ended up with a real problem.
"If you look at the points Fulham had because of him... Can you imagine the scenario if Clint hadn't been here over the last three or four years?
"Our strikers never used to score a lot of goals. Now I think we're fine, but we don't have Clint. If Clint had stayed I think we would've been in the top 10.
"We sold our top scorer and he (Dempsey) was the top scorer for a couple of years. Last year he scored more goals than any year before - and Dembele was my best player, arguably.
"We had to build a new team. If everyone is fit I still feel we're not far from the quality we had last year."
Jol expects his strikers to score, with Dimitar Berbatov carrying much of the weight of expectation this term, but Dempsey's goals from midfield were a bonus.
"I feel we will score more goals than before, but we need the important and vital goals," Jol added. "Clint Dempsey scored all our winners last year."