Joe Schmidt has not ruled out Ian Madigan holding off Paddy Jackson to start the first test against South Africa tomorrow week.
"The big advantage for Paddy is his form and regular game time at the end of the season," said the Ireland coach.
"But, Ian's trained incredibly well in the two trainings that we've done.
"Those are the decisions that we'll make at the latest by Thursday," he said, ahead of the first test in Johannesburg tomorrow week.
The last thing Schmidt will look for is a pale imitation of Sexton.
The odds must favour Jackson's consistent growth as a leader for Les Ulster.
"If it is Jacko,' it's a real opportunity for him to fill those big boots, that big jersey, but also be himself.
"I'm not going to ask Paddy Jackson to be anyone other than the player he's comfortable being because that's a good player.
"That's a player that can be effective at test level."
The shoulder surgery for Jonathan Sexton and withdrawal of Luke Fitzgerald (knee ligament) and Rob (hamstring strain) and Dave Kearney (calf strain) has meant the return of Madigan and Craig Gilroy.
In addition, there is the doubling of the uncapped to four with the promotion of Connacht's Matt Healy and Tiernan O'Halloran.
"If there are guys missing, I'm going to know more about Tiernan, more about Matt in a week-to-week environment," said Schmidt.
It could well be that Robbie Henshaw comes in for Rob Kearney at full-back with Jared Payne seen as a cornerstone of the Irish defensive system.
"Robbie has been playing some good rugby at full-back as well and that would st ill allow us to use Jared to control our edge defence," said Schmidt.
This would open up a debate about the No 12 shirt where the hugely talented Stuart Olding and the in-form Luke Marshall may come into that equation.
"It'll be a big place when the big South African forwards start coming around the corner," Schmidt said with a 'rather them than me' demeanour.
"There's some pretty unpleasant sights for those inside backs at that stage."
All of a sudden, mission improbable looks almost impossible.
IRELAND SQUAD FOR SOUTH AFRICA.
Forwards (18): R Best (capt), F Bealham, S Cronin, U Dillane, T Furlong, J Heaslip, I Henderson, D Kilcoyne, J McGrath, J Murphy, S Reidy*, M Ross, Q Roux*, R Ruddock, D Ryan, CJ Stander, R Strauss, D Toner;
Backs (14): K Earls, C Gilroy, M Healy*, R Henshaw, P Jackson, I Madigan, K Marmion, L Marshall, C Murray, T O'Halloran*, S Olding, J Payne, E Reddan, A Trimble.