Leinster look to Leavy to lead the charge of the back row
Lancaster warns how Saracens will use England's failure to drive their success
This game can change in the blink of an eye.
The pecking order of Leinster's back row at the beginning of the season would have read something like Sean O'Brien, Jamie Heaslip, Rhys Ruddock, Josh van der Flier, Jordi Murphy, Jack Conan, Dan Leavy and Max Deegan.
Eight months on, Heaslip has retired, Van der Flier's season is over, Jack Conan is carrying a knock to his knee and Sean O'Brien has to train fully today to have a chance of playing against Saracens on Sunday.
The road through the Six Nations has led to a change of order, for one reason or another.
Now, Leavy stands out as the only player guaranteed to start in the back row.
Of course, the 23-year-old flanker, blessed with bullet-proof self-confidence, would never have agreed with the pecking order in the first place.
Then again, none of the others would, or should, either, because they all believe they have the tools to be one of the three.
This is where the objective opinion of the coaches come into play.
Stuart Lancaster is well-versed in how the options can move from 'who to leave out?' to 'who to put in?' as the brutal nature of the game takes its share of victims.
"There is no doubt you were sat there at the start of the season thinking Jesus, we have an unbelievable number of back-rowers," he said.
"Then, suddenly, you lose Jamie, lose Josh Van der Flier, not that long ago, one or two knocks now."
The current champions have not escaped untouched and there are just as many questions hovering over their arsenal.
"I don't think it's any different from some of the challenges that Saracens are facing," issued the former England coach.
"They have Billy (Vunipola), who will be nearly back - who knows if he will be or won't be? - they lost Calum Clark to a hand injury not that long ago.
"They've brought in Blair Cowan, played at seven at the weekend. It's the same for both teams really."
It just goes to show how the landscape has altered from Leinster qualifying as the top seed and Saracens as the eighth seed in January.
Make no mistake, this is a 50-50 contest, with Sarries hunting the three-in-a-row and Leinster looking to turn back the years.
Like James Ryan, Leavy spent the seven weeks of the Six Nations proving his value as a legitimate international openside.
It is something Lancaster always knew was inside of him.
"I've always thought he was going to be there or thereabouts for Ireland, in terms of his quality," said the senior coach.
"When he finally got his opportunity, I think he demonstrated the type of player that he is.
"He's very good over the ball, incredibly strong and powerful, and, particularly for me, he made very good decisions at the breakdown."
This is a common theme that surfaces about Leavy, the where and when to go after the ball.
It looks like the ex-Ireland U20 captain has figured it out. It is now a matter of chiselling away at it, not falling back into old habits.
"He looked composed and controlled in that environment," said Lancaster.
"It's obviously very intense. There's a lot of scrutiny on players in those games.
"I thought he dealt with it really well - alongside the other players who were making their early starts in the Six Nations."
It would be naive to presume the Grand Slam can be tucked under the arms of the Leinster players and used to carry them past the English club.
For instance, the disruption to Saracens caused by the Six Nations pales in comparison to the 14 Irishmen from seven starters and seven replacements at Twickenham.
"I think it'll work both ways," he cautioned.
"There's be a spring in the step and the confidence that winning a Grand Slam brings.
"That said, every game starts from zero."
The positivity Leinster can put to use is countered by the failure that will propel the likes of Maro Itoje and Jamie George.
"Lads will leave a situation where they have not got the outcome they wanted and the motivation is to go back to the club and play.
"Mark McCall has done a brilliant job at creating that sense of culture and identity that drives their mentality."