The 27-year-old blindside underwent surgery in early December to repair tendon damage sustained in a RaboDirect PRO12 match against Edinburgh in November and a review of the ankle has now shown that further treatment will be required.
Ulster are yet to confirm whether the 2009 Lion will need another operation but he is now quickly running out of time to prove his fitness before Warren Gatland names his Australian touring party at the end of April.
"Stephen was reviewed this week (Tuesday) as part of his ongoing rehabilitation programme. In that review it was discovered that, although Stephen has made excellent progress in his recovery, some minor issues remain," read an Ulster statement.
"As a result, the specialist medical-staff treating Stephen are considering several options to ensure that he makes a full and complete return to play.
"A decision on a course of action will be made in the coming weeks."
Ferris has played very little rugby this term, with the ankle problem that ruled him out of Ireland's autumn series and the recent games against Wales and England just the latest in a long line of injury issues to have plagued him in recent months.
Calf damage prevented him from participating in Ireland's summer tour of New Zealand and a back complaint hampered his early involvement in this season's PRO12.
Injury also scuppered his hopes of a Lions Test spot in South Africa four years ago as his knee gave out to bring his tour to a premature end.
Ferris was one of the stars of the initial weeks of the last Lions adventure, scoring tries in each of his two appearances against the Golden Lions and the Cheetahs in Johannesburg and Bloemfontein, and he had been heavily tipped to achieve similar success with Britain and Ireland's elite this summer.