It had been hoped that Ferris would return to the pitch in time for the crunch Heineken Cup quarter-final with Northampton Saints but he requires further exploratory surgery.
"Having undergone a period of treatment and rehabilitation for the left knee injury sustained during Ulster's match against Aironi in January, Stephen was reassessed by his surgeon on Tuesday and a decision was taken for him to undergo arthroscopic surgery," read an Ulster rugby statement.
"It is hoped that following this procedure a definitive diagnosis and prognosis for return to rugby will be reached however this means that Stephen will not be available for Ulster's Heineken Cup quarter-final match against Northampton Saints on 10th April."
Ferris is now facing a battle to be fit for the semi-finals of either or both the Magners League and Heineken Cup - should Ulster qualify.
But there is some good news for the Irish province with Ian Humphreys, BJ Botha and Andrew Trimble all returning to the Ulster squad to face the Scarlets on Friday night.
Ferris made a huge impact during the 2009 Lions tour of South Africa with two tries in two games before his trip was cut short due to injury.