Umar will undergo surgery and will miss the forthcoming Test series against South Africa and the one-day series against New Zealand in November.
"He has suffered double injuries in his left knee which flared up during the first two Tests while an old injury in the right knee has also recurred," PCB spokesman Samiul Hasan told reporters.
The 22-year-old left-handed batsman scored 75 and 43 in Pakistan's second Test win over Bangladesh in Peshawar last week, but missed the third Test through injury.
"Doctors have said he will take 8 to 10 weeks to recover from post surgical rehabilitation," added Hasan.
