Commenting on O'Connell's recurrence of a bulging disc problem, Ireland team manager Michael Kearney said: "There's been no timeframe put on his return to play at this stage. It's a question, I think, of just monitoring it as we go along and see where it ends up.
"There's no fixed date or estimate as to when he actually will return to play. It is out there in the public domain that he's had a couple of (anti-inflammatory) injections. He had a further injection on Friday.
"It will be managed for the next while and the hope is that it can settle down over the next few weeks.
"Certainly the nature of the injury means that it's a question of wait and see. It certainly isn't a black and white situation in terms of knowing the exact prognosis for what you do from this moment on."
Ferris' recovery from his injury has been 'slow' according to Kearney, who explained: "He has significant ankle ligament damage and is continuing to receive intensive treatment.
"But at this stage, we don't expect him to be available for the game against Argentina on Saturday week."
Of the injuries emerging from the defeat to South Africa, scrum half Eoin Reddan is a doubt for the Fiji match due to a bruised ankle.
Forwards Cian Healy and Donnacha Ryan have both been passed fit following post-match concussion testing, while Reddan's half-back partner Jonathan Sexton is recovering well from knocks to his shoulder and hand.
The Ulster quartet of Paul Marshall, Craig Gilroy, Tom Court and Roger Wilson have joined up with training at the Irish camp in Carton House. Munster prop Stephen Archer has also been drafted in along with Leinster lock Devin Toner.