IT IS already make or break time for coach Simon Easterby's Scarlets. This is part of the magic and malevolence of the Heineken Cup.
ONE slip and you're almost on your backside. Almost, mind you. The Welsh club can still turn it around. They have a better squad this year than any other before it.
While Leinster are not at the same fork in the road, a defeat in west Wales to accompany the hiding they took there in the Pro12 League would wound them deeply.
The cavalry of players out injured is lessening all the time for Joe Schmidt. The question is: will Europe still be an option when Sean O'Brien et al return to their posts?
The Scarlets started out as world-beaters this season, posting a bloodless 45-20 victory over Leinster in running up 87 points in their first three matches to stand at the top of the Pro12.
Since then, there has been an alarming dip, prompted by a significant defeat (23-16) to the Ospreys at home in front of 15,000 spectators. Treviso also put paid to them (22-20) in the last minute in Italy.
In fact, the problems have intensified rather than receded, Clermont Auvergne completing a troubled run of three losses in their four outings.
Scarlets are one down with five to go in Europe. This is not to say that they are vulnerable. Sure, they are wounded. But they are dangerous, so dangerous.
The list of rapiers in the backline is enough to make an Eskimo sweat, with wing George North and centre Jonathan Davies front-runners for British and Irish Lions Test selection.
Leinster have to start well, consolidate their position and wait for The Scarlets to push the passes that don't stick. It is a game of patience, not necessarily going in favour of the one with the best cards to play.
Coach Schmidt will put real stress on his players to minimise mistakes, cut out the small errors that, inevitably, kill continuity and the momentum that comes from it.
The champions have some gold dust of their own to spread all the way across the argument. They also have the knowledge that they have not fired yet this season.
There would be no better time to let their light shine.
Scarlets v Leinster, Parc Y Scarlets, tomorrow 1.35 (Live, Sky Sports 1)