Cullen's outfit can't afford to play fast and loose
Guinness PRO12 semi-final Leinster v Scarlets, RDS, Live TG4 & SS1 (Tonight, 7.45)
The PRO12 has always been the unloved League compared to the European Cup.
That is why the semi-final is not sold out for tonight at the RDS.
It is second-best in every conceivable category that matters, except one.
There is a reasonable argument to be made that it was the competition that spawned Leinster, not the Heineken Cup.
It was known in a previous carnation as the Celtic League when Leinster won its first trophy against Munster in the 2001/2002 season.
It was the steady blocks of confidence built from there, and the subsequently re-named Magner's League in their 2007/2008 League title, that enabled Leinster to foster the winning habit to make the European breakthrough in 2009 - at the first time of asking in that major final.
This season, Leinster ended their European adventure with 'what ifs?' crowding up the back of their minds.
Then, they failed to bring the required intensity or accuracy to Ulster in the last round of the PRO12 which put them on course for The Scarlets tonight rather than The Ospreys tomorrow.
"This is a different game," said Cullen, intent on reading from a clean slate. "We just need to make sure we've learned the lessons from the games where we haven't quite hit the right notes this year.
"We can also take confidence from when we do get things right that there is a lot of threats through the team.
"You look back to the semi-final against Clermont, how we managed the breakdown, how we managed the first 20 minutes, in terms of being composed.
"When you play in these big games, it is important that you remember some of the lessons from the review that has gone on in the club."
Cullen's consideration in selection has removed the contest at the breakdown between, statistically, the two best ground men in the League as the excellent Josh van der Flier has edged out Dan Leavy to counter James Davies.
"All the time you're looking for balance in the game and the opposition you're going to play," issued Cullen.
"The form of guys, physically how they are, the stage of the season, what's going on in the rest of their lives.
"You are making those calls and assessments all the time," he said. "Josh has been a very steady performer for us, the same as the rest of those back-row guys.
"Dan has come in and done incredibly well and it's great to have that luxury.
"But, we've got a strong group that will start and we want to have a strong group to finish as well."
This is where Leavy and the under-cooked Seán O'Brien will come into the equation in the 6/2 split between forwards and backs which suggest strong-armed tactics by Leinster.
They have to smash into The Scarlets, deny the Welshmen time and space and the ball.
The visitors' under-powered front-five have to be devoured or a trigger-happy three-quarter line could become Leinster's worst nightmare.
Cullen does not want to play fast and loose with a club that has already beaten Toulon and drawn with Saracens this season.
This narrows the focus in towards the reliability of props Jack McGrath and Tadhg Furlong, locks Hayden Triggs and Ross Molony, preferred to Devin Toner for this task, and the hard-charging of Rhys Ruddock and Jack Conan.
Hooker James Tracy must take his throwing up to the level of his loose play to put the seal on a level of control which Jonathan Sexton will convert one way or another.
Leinster: J Carbery; A Byrne, G Ringrose, R Henshaw, I Nacewa (capt); J Sexton, L McGrath; J McGrath, J Tracy, T Furlong; R Molony, H Triggs; R Ruddock, J van der Flier, J Conan.
Scarlets: J McNicholl; L Williams, J Davies, S Williams, S Evans; R Patchell, G Davies; R Evans, R Elias, S Lee; L Rawlins, T Beirne; A Shingler, J Davies, J Barclay (capt).