D'Arcy has had surgery on his right ankle following Leinster's Heineken Cup Final win over Northampton at the end of May, with the injury having not responded sufficiently to a course of rehabilitation.
Ferris hasn't played since helping Ulster reach the European quarter finals in late January, with the 2009 Lion missing all five of Ireland's RBS 6 Nations fixtures and 12 competitive games for his club.
And while Ireland team manager Paul McNaughton remains optimistic of the duo's chances of making the global gathering, their involvement in the August friendlies is in serious doubt.
"If there is a likelihood of guys not playing before the World Cup then it's those two," said McNaughton.
"It's a preference that they have some game time before they go, but we'll never say never on anybody.
"We're hopeful that they may be ready for game time, maybe in match three or four of the summer series.
"Gordon is tight to make the World Cup. He recovers quickly and is running while bearing his full weight, so is making progress."
There was better news over another trio of Lions, however. Rob Kearney, Geordan Murphy and Jerry Flannery have all returned to training following long-term injuries and are available for selection against Scotland this Saturday.
"Flannery, Kearney and Murphy are ready to go," added McNaughton.
"In general, the injury situation has been getting better over the last couple of weeks, but we have five matches and need to give guys game time."