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."
Jol recruited Dejagah as a relative unknown from Wolfsburg last summer and he has been pleased that his hunch paid off, with the 26-year-old making 24 appearances for the club before injury struck.
"I'd like to play well-known famous players but it's not always possible. There are other clubs scouting the stars so we have to come up with a different idea," Jol said.
"Sascha Riether (another player brought in from Wolfsburg) was the same.
"There are a lot of other players in football that you have to take when they are not doing well if you want to get them cheap - but they did quite well for us here.
"So we were happy with Dejagah. He was cheap but he played for a good club, they were champions in Germany and I knew him from that."
Jol is waiting to discover whether Enoh will be fit for the trip to Villa Park after he limped out of Fulham's defeat to Newcastle.
"We will do an assessment tomorrow. He could probably play but we are not sure yet," Jol said.
"I feel we need another win because you never know. Realistically we are fine but if everyone else starts winning in the last six or seven matches then you could have a problem.
"Hopefully we will get a couple of results in the next week or so.
"Overall we have done well away from home. There have maybe been too many draws and even at Newcastle we could have done better. It is disappointing to concede in the 93rd minute.
"They [Villa] have got quality. They have (Christian) Benteke, (Gabriel) Agbonlahor, even (Darren) Bent on the bench. If you look at their results, especially at home, they are OK."