American international forward Dempsey, who scored 60 goals in 224 appearances for the Cottagers, was sold to Tottenham last August in the same month that midfielder Mousa Dembele also joined Spurs and Jol says the club are still adjusting to life without the pair.
"Clint Dempsey always made the difference," said the Dutchman, whose side face Norwich on Saturday. "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.
"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.
"I feel we will score more goals than before, but we need the important and vital goals. Clint Dempsey scored all our winners last year."
"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."
Having taken 28 points from 25 games to leave them 13th in the table, Jol admits his expectations for this season have been downgraded from their pre-season aims.
"Our target was first to be in the top 10 and now the target is to stay in the league. Although we had those players last year the difference is still two points."