World Cup glory is sure to make Johnny Sexton Ireland's greatest athlete.

Johnny Sexton's Irish side head into Saturday's World Cup match against Scotland on the back of 16 straight wins. Photo: PA/Andrew Matthews

When Andy Farrell casually mentioned Johnny Sexton «almost getting his head ripped off» by James Ryan during a spirited training session earlier this week, you could almost hear the sharp gasp from the assembled press.

Ireland can become the number one team in the world. They may head into Saturday's Group B final against Scotland on the back of 16 straight wins, with more depth and quality in their ranks than ever before. But it would not be an exaggeration to say that they are still afraid of something unpleasant that could happen to their captain and mascot.

In almost every other position in the team, Ireland now have viable alternatives. Tadhg Furlong would certainly be a big miss at tighthead. But Finlay Bealham performed brilliantly as Leinster's stand-in at this year's Six Nations. Pharrell has locks and back ridges protruding from his ears. He has two world class prostitutes. Two halves of world class action. There are so many midfield options that almost no one has noticed that Robbie Henshaw may be out of the tournament with a hamstring strain.

But if Sexton so much as sneezes, just watch the reaction across the Irish Sea.< /p>

This is no disrespect to Ross Byrne or Jack Crowley, both excellent players. But with 21 and 6 caps respectively, they miss Sexton's experience, his leadership qualities, his ability to lead his team in all areas, both on and off the field.

Indeed, Ireland's failure to produce Sexton's successor is only partly due to bad luck (Joey Carbury's injuries) or to causes beyond their control (Paddy Jackson's trial and acquittal). This is mainly because Sexton was so good, such an integral part of their style of play, that when he is gone, the quality of play drops so clearly that no one can make the reserve position their own.

It got to the point where Sexton was being talked about as more than just Ireland's greatest rugby player — his achievements of leading Ireland to multiple Grand Slams and victories over New Zealand home and away eclipsed the likes of Brian O& #39;Driscoll, Willie John. McBride, Mike Gibson and Jackie Kyle — but the point is the sporting GOAT of Ireland. Who else is in the conversation? Roy Keane? Katie Taylor? Ruby Walsh? Rory McIlroy? A. P. McCoy?

If Ireland win this World Cup, you can probably put a fork in this debate.

It is already clear to Farrell that his captain is Ireland's greatest ever player. He said this after Ireland won a Grand Slam against England on St Patrick's Day in Sexton's last Six Nations appearance earlier this year. O'Driscoll said he wouldn't argue with that, especially if Ireland won the tournament. “It’s none of my business what people think of me or Johnny,” he said after Pharrell’s comments.

“These conversations will always continue. Some people will say Paul O'Connell, people will say Mike Gibson, some people will say Jackie Kyle. But I guess based on what Johnny has achieved so far, if he goes and wins the World Cup, it would be pretty hard to argue with him.” One suspects that for this to happen, Ireland will need to clean up Sexton and sack him for a few more weeks.

As usual, the man himself downplayed his value to the team on his press conference this week. Asked how he was preparing to face his opponent on Saturday, Scotland's equally talismanic (in his own way) midfielder Finn Russell, Sexton was almost dismissive.

“As I say, every time I’m asked about the other half, we never pit ourselves against each other,” he responded. “These are our teams. As an outfielder you have to rely on the guys around you and we rarely run into each other or go toe-to-toe. But we are both responsible for the teams we play for. He is an outstanding player with a full bag of tricks. We had to fully prepare for him.”

That includes risking his neck every practice, even if it means exposing his head to a loaded 116-pound lock. Farrell just laughed when asked if he had ever been tempted to wrap his captain in cotton wool. “No, no,” he replied nonchalantly. “Wrap it in cotton wool? No. Johnny has been used to playing against teams all his life who want to put him under pressure, and if we can implement that in our sessions, it will help him along the way.”

Internally, however, he must wince every time sees Sexton getting hit. If he had not done this, he would not be human. Farrell knows better than anyone how important Sexton is to Ireland's progress.