The Hearts winger was injured when his shot was blocked by John Potter during Saturday's victory over St Mirren. He was sent for an MRI scan last night which confirmed ligament damage and he is likely to be sidelined until after Christmas.
Csaba Laszlo, the Hearts manager, will consult club medical staff later today for an exact recovery time for Mikoliunas but, speaking to the Evening News, he said: "I don't think he will be able to play football for some time. It is a very serious injury.
"He struck the ball and his leg was blocked and this is what caused the injury.
"I don't know at the moment if he will need an operation, we will speak to the doctors.
"The knee is swollen and we will talk more today after the MRI. We don't know exactly how long he will be out but he has a serious injury, definitely, and he cannot play."
Laszlo is preparing for tomorrow evening's visit of Hamilton to Tynecastle without Mikoliunas and the suspended Michael Stewart, however, Marius Zaliukas and Christos Karipidis will return to the squad.
The Lithuanian missed the victory at Love Street whilst serving a one-match suspension and Karipidis has recovered from a minor muscle complaint.
Laszlo continued: "Zaliukas is ready to come back and he can play against Hamilton. Karipidis could maybe have played against St Mirren but we did not want to risk something more serious with his injury.
"He had a knock on his muscle and it was very painful for him, but not something serious.
"He trained yesterday and again today and we hope everything will be okay from now on. He is in the squad."