The 2009 Lion had hoped to make it back in time for Ulster's end-of-year run-in in both the Heineken Cup and Magners League.
But Ferris missed the crucial European quarter-final defeat to Northampton at the beginning of April and will now sit out Friday's domestic Play-Off semi-final with Leinster.
As well as being unavailable for the last 11 club games, Ferris missed Ireland's entire Six Nations campaign.
He hopes to be back fully fit to take part in his country's preparations for the World Cup but the Ireland medical team have decided he is not yet ready to return to the field of play.
Even if Ulster upset the form book and beat Leinster in Dublin, Ferris won't feature in the Magners League Grand Final on May 28.