Johnny Sexton must start against France if fit, according to Brian O’Driscoll.
The out-half missed the first two rounds of the Six Nations with a calf injury but is expected to train fully with the squad tomorrow.
Paddy Jackson has been an able deputy and former Munster and Ireland hooker Damien Varley told the RTÉ Rugby podcast that even if the ex-Racing 92 player is fit, it is the Ulster fly-half who must start.
However, O’Driscoll, who retired after winning 133 Ireland caps, is adamant that his old provincial team-mate should get the nod.
“If Sexton is fit he starts. I've played with both,” he wrote on Twitter.
"Jackson is a good player and hugely improved but Sexto is world class."
"No doubt he's been very good. Just think Johnny brings more."
O’Driscoll added that one of the traits of Sexton’s game is an ability to step back into the firing line at full pace, even after a prolonged spell on the sidelines.
“[There’s] no doubt about that."
Full-back Rob Kearney is also expected to take full part in tomorrow's session.
Scrum-half Conor Murray's was rested last week due to a tight abductor, but is not rated as a doubt for Saturday's game with France (4.50pm, live on RTÉ Radio 1 and RTÉ2).
"Everybody looks good to fully train tomorrow," said Ireland team manager Dean.
"Johnny Sexton will play a part in training today, but will fully train tomorrow. Hopefully when he comes through that, he'll be fine for the weekend.
"Rob Kearney continues to make progress. He'll do some contact work today and he'll fully train tomorrow.
"If you talk to Johnny and Rob they'll both say that they're 100 per cent fit and ready to go, so we just need the medics to pass them and they need to prove to us that they are.
"Conor Murray's workload was managed in Monaghan last week. It was a difficult week for the players last week, but I'm happy to report that the outlook is positive.
"Conor was tight, so we thought it prudent to call up Kieran Marmion. He trained with Connacht on Thursday, with us on Friday and he was rested at the weekend."
Josh van der Flier could miss the rest of the Six Nations after suffering a shoulder injury, though the Leinster flanker's absence should be offset by Munster back-rower O'Mahony's return to fitness.