Dejagah suffered an ankle ligament rupture when he collided with an advertising hoarding in Fulham's 3-2 victory over QPR on April 1 and he will undergo surgery next week.
Fulham, currently 10th in the table, head to Aston Villa on Saturday with Jol insistent they need one more win to be absolutely certain of avoiding relegation.
But they will head to the Midlands without Dejagah and potentially the Cameroon international Eyong Enoh, who faces a late fitness test on an ankle injury.
"Ashkan Dejagah needs an operation on his ankle ligament. That will be next week," Jol said.
"It's disappointing. When you come to England you have to cope with the Premier League and that is what he did.
"We needed his pace and his ability on the ball. He was man-of-the-match a couple of times.
"We were positive after last week's match and thought it would be two weeks or 10 days but it seems that there is a little rupture so he needs surgery."