Dejagah requires an operation to repair ankle ligament damage suffered when he collided with an advertising hoarding in Fulham's 3-2 victory over QPR on April 1. The Iranian will go under the knife next week and he will not play again this campaign.
"Ashkan Dejagah needs an operation on his ankle ligament. That will be next week and I think he will be out for the rest of the season," Jol said.
Jol is waiting to discover whether Cameroon international midfielder Eyong Enoh will be fit for Saturday's Barclays Premier League trip to Aston Villa.
Enoh limped off in Fulham's defeat at Newcastle last week and Jol said: "We will do an assesment tomorrow. He could probably play but we are not sure yet."
Fulham head to Villa Park with Jol believing the club need one more win to guarantee their top flight place for next season.
The Lilywhites face a Villa side battling for their own survival, with Paul Lambert's men just two points above the relegation zone.
"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," Jol said.
"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) Beneteke, (Gabriel) Agbonlahor, even (Darren) Bent on the bench. If you look at their results, especially at home, they are OK."
Source: PA
Source: PA