Paddy Jackson is on standby to regain the Ireland No 10 jersey for Saturday's clash with Samoa after Johnny Sexton sat out training yesterday.
The Ulster fly-half looks set to get the nod ahead of Ian Madigan if Sexton does not shake off the hip injury he sustained in training last week.
The Racing Metro pivot strained the joint at kicking practice last Wednesday and, although he trained on Thursday and Monday with Ireland and took his place on the Parisians' bench on Saturday, he has been unable to relieve the tightness in his hip.
Joe Schmidt warned last week that yesterday's training session would represent a cut-off point for any player struggling with injury and was boosted by the fact that Paul O'Connell, Sean O'Brien and Robbie Henshaw came through their first training sessions with the squad yesterday.
But Sexton's inability to tog out at Carton House puts his participation in doubt as a result and, while it is believed he is pencilled in to start the clash with the islanders, the door is now open for Jackson to step in if needed.
The 21-year-old finished last season behind Ian Madigan in the pecking order, but his time on the pitch with Ulster has helped to leapfrog him over the Leinster man who has had a frustrating start to the season at club level where Matt O'Connor has opted to start New Zealander Jimmy Gopperth in big games.
Meanwhile, O'Connell remains a doubt despite coming through training yesterday and the uncertainty over his fitness could undermine his candidacy for the captaincy for the November tests.
Brian O'Driscoll, Jamie Heaslip and Rory Best remain in the frame for leading the side out at the Aviva Stadium on Saturday.
Given the clash is Schmidt's first game in charge and with Australia and New Zealand coming hot on the Samoan's heals, O'Brien – who has overcome his ankle injury – said it is important not to get off to a losing start.
"Obviously we don't, winning is the priority," he said.
"If we perform well and we do our own jobs, hopefully it leads to the win, bar something coming out of the blue or they get a breakaway or whatever in the last minute.
"Hopefully if our performance is good enough, the win should be there."
Forwards coach John Plumtree revealed the Irish have held off confirming their squad internally for Saturday's game.
Schmidt had been hoping to have already named the line-up to his players, but Plumtree has now admitted the Ireland coaches will keep monitoring O'Connell's continued progress past his calf muscle problem.
Should he be passed fit O'Connell will have a strong claim to the Ireland captaincy, due to be confirmed when Schmidt announces the Samoa match squad on Thursday.
Plumtree remains unsure whether 34-year-old veteran O'Connell could last the full course against Samoa, but the indications are he will be involved.
The former Natal Sharks coach said: "Paul O'Connell ran, Sean O'Brien ran. So they have been struggling a little bit.
"We haven't named the squad yet. It will be a medical decision based on the fact that he (O'Connell) could do 80 (minutes).
"Whether he can go 80 or not, I'm not sure. We'll think about it and see what we are doing.
"He's our most experienced lock. He's been the glue in that pack for a fair while.
"He's important; he's still keen, still enthusiastic and still loves playing for Ireland, so he's important."
Plumtree unwittingly found himself on a reconnaissance mission at Blackrock College this week while on the school run.
The ex-Wellington Lions coach said he did not need to watch a Samoa training session to know what Ireland have in store for them this weekend.
Plumtree said: "They were training at my son's school so when I was going down to pick him up I was watching their training for a bit.
"I wasn't spying on them! I didn't know it was going to happen.
"But they were well-organised, training was good and their work ethic was particularly strong.
"In the past Samoa relied on individuals but I was in South Africa when they were playing over there against Scotland, Italy and South Africa. And they were really playing as a team.
"Defensively they were well organised.
"They have really stepped up and I guess it is really because of the individuals they have playing around the world now, being exposed to different teams and cultures.
"They are a real threat now because they are not only dangerous because they are individually good - they play as a team."