Sexton faces his toughest battle to be the Lions' perfect 10
Not since Ronan O'Gara made the transition from his great rival to his great friend has Johnny Sexton faced a real battle for his position.
This morning, he wears the Lions No 10 jersey in the opening game against the Provincial Barbarians, but there is no guarantee that he'll retain it for the three-Test series against the All Blacks.
Four years ago, Owen Farrell was the apprentice to the master out-half whose pre-eminence was clear from the outset. Now, the youngster has back-to-back Six Nations and Champions Cup titles under his belt and believes he is ready to lead the Lions backline.
They could yet link up together, given the 25-year-old's capacity to play at inside-centre - where he lines up for England - but for now it looks like a battle for one slot, with Wales' Dan Biggar also in the mix.
It is, Sexton says, the toughest battle he's faced for any jersey.
"I'd say so, yeah, Owen's a world-class player," he said.
"He's had some great success over the last couple of years.
"He's coming in as favourite to start somewhere, so it will be up to me to try and prove that I can still play that role.
"My goal at the start of the year was to get into that squad 'Gats' picked.
"The Test team will be picked on form, I think, so to try and make as good an impression as I can and try to play to my potential.
"If that gets me into the team or the squad, or not in the 23, if I did my best I can live with that.
"I think it's a clean slate. I don't think they're going to say 'oh, he did well four years ago, so we're going to put him in'.
"I think it'll depend on form from here to the first Test and how well you do," he added.
Gatland's decision to select him for the opener was born of a need to get games into Sexton's legs.
The player says he's been fully fit, insisting that Leinster chose to race him lightly in the closing weeks of a season that ended with double semi-final disappointment and a harrowing defeat to Scarlets.
"That was the coaches' decisions," said Sexton.
"It was at the start of the season I struggled to get on the pitch. In terms of game-time for the whole season, when I was on the pitch I was happy with how I played, but at the end of the season I was fit and ready to go.
"Obviously it wasn't ideal. We had a quarter-final straight after the Six Nations, so the guys didn't want me to play before that and didn't want me to play before the semi-final.
"But I was training on the pitch the whole time. Apart from a few bumps and bruises you always carry, I've been fairly injury-free and I'm ready to go now," he said.
He is in familiar territory, as the coaching ticket is the same as the one that led the series win over Australia four years ago, while he is a veteran of two tours of duty in New Zealand.
What's different about him, however, is his experience of finally beating the All Blacks last November.
And he is keen to drive home the message to his squad-mates that they must take the game to the world champions when they meet in three weeks' time.
"There's ways that you have to play against them," he said.
"We stuck to the game-plan in Chicago and in the Aviva, we caused them trouble, but when we went away from that we struggled against them.
"You can't sit back against them, that's the one thing I think the Irish guys might try and influence with the other guys, that you've got to keep attacking no matter what the scoreline.
"You saw that in Chicago more than anything. When we sat back we conceded two or three tries, and then flipped the switch again and started to attack and we got some rewards.
"Remember Robbie Henshaw's try and how that came about, attacking off the scrum and putting the ball in behind them?
"So it's not just attacking with ball in hand, it can be an attacking, kicking game and defence as well. So that's the one big thing," he said.
"It's a different time of the year, the middle of their season rather than the end.
"You don't get the best All Blacks in November when they're tired; it's around the summer tour where we've struggled against them in the past, so we're going to have to find another gear to get past them this time," he added.