Dejagah was hurt colliding with an advertising hoarding against QPR on 1 April and he will need surgery to correct the problem.
The Iranian will go under the knife next week and Jol does not believe he will be available again until the start of the new season.
"Ashkan Dejagah needs an operation on his ankle ligament," Jol said.
"That will be next week and I think he will be out for the rest of the season."
Jol is also unsure whether Cameroon midfielder Eyong Enoh will be available for Saturday's trip to Aston Villa after hurting his ankle against Newcastle.
The Dutchman added: "We will do an assesment on Friday. He could probably play but we are not sure yet."