No one will follow a leader who goes backwards.
With Cian Healy still short of where he needs to be, Jamie Heaslip deemed unnecessary due to the hot form of Jordi Murphy and Iain Henderson on the bench, the role of leading by forward motion will fall to an under-cooked Carlow man.
"Sean O'Brien is a leader by the way he plays the game," said Joe Schmidt.
"People tend to follow him because he tends to go forward."
For all of second rows Paul O'Connell and Devin Toner's work rate, they are not guaranteed to crash through the gain line, get in behind a spread defence and cause panic.
If O'Brien can get well beyond there two or three times, it could make all the difference as Italy back-pedal and Ireland's quicker men out wide stream forward onto the ball.
For sure, The Irish Wolfhounds-English Saxons was a trial in the traditional sense. Kieran Marmion and Ian Madigan didn't do enough to convince Schmidt.
O'Brien stood out for as long as he could stand up. The brutal truth is the flanker's 60% is better than most players' 100%.
"We certainly didn't necessarily see Sean starting this week. But, we felt he did well in The Wolfhounds game," said Schmidt.
"There's always a degree of risk in bringing a player like that off the bench because if they run out of steam, you've got nowhere else to go.
"Whereas if Sean can give us a great 50-60 minutes, then I think that will help put us into the game against what will be a really tough side."
This is where the versatility of Murphy and Peter O'Mahony, two decent carriers in their own right, as men who have played all across the back row.
Their accuracy as ruck cleaners will have to be pinpoint and they will have to get to the breakdown sharply.
In addition, the mere presence of O'Brien will draw the eyes of Italian defenders, leaving a metre or two more for others to work in.
Of course, the Tullow Tank will empty himself for the cause. That is what he does. That is what he did on his return. "Going to Italy for the first Six Nations game, you know that's a massive ask for someone who hasn't played in whatever length of time," he cautioned last week.
The time away from Ireland has fuelled his determination to get back before the Six Nations.
He has had time enough to sit and brood about what he could do, what others have been doing.
"What have I been looking at? I've been looking at lads in my position, playing very well," he said.
"Obviously, I've been looking at the way they've been playing. I've been following it, I've been in touch with Joe, I've been in touch with the lads.
"You know I'm asking questions. I've been trying to keep up to date as much as possible. So it's been grand and it's been an easy enough transition to be honest because most of the stuff isn't too different from what we've done before with Joe.
"You still remember a lot of it."
It won't have gone unnoticed how O'Brien completed three turnovers in the first 13 minutes for The Wolfhounds. The man can do it all, on both sides of the ball.
Italy: A Masi; L Sarto, M Campagnaro, L Morisi, L McLean; K Haimona, E Gori; M Aguero, L Ghiraldini, M Castrogiovanni, J Furno, G Biagi, A Zanni, F Minto, S Parisse (capt).
Ireland: R Kearney; T Bowe, J Payne, R Henshaw, S Zebo; I Keatley, C Murray; J McGrath, R Best, M Ross, D Toner, P O'Connell (capt), P O'Mahony, S O'Brien, J Murphy.