Leinster have five days to find their form. Or else.
The three-time European Cup winners looked like a pale shadow of their former selves as they ran out of ideas and road at The Stoop on Saturday.
The list of injury outsiders looking in would make any club creak. Forwards Sean O'Brien, Cian Healy, Richardt Strauss, Marty Moore, Dominic Ryan, Kevin McLaughlin and Shane Jennings are nothing more valuable than helpless bystanders.
Backs Fergus McFadden, Dave Kearney and Ben Te'o reduce coach Matt O'Connor's options further in an overall total of nine missing internationals.
This is happening at a time when Harlequins have to handle life in The Premiership with a much smaller squad, typically using around 26 players and are relatively unscathed.
This is the professional game. There is no room for sentiment or excuses.
There is only the squeezed time of five days to fix what went wrong with what they've got.
1. ITS' A FIGHT
First and foremost, rugby is a match of will and muscle at its core.
Take away the gain line from the opposition and you are halfway there.
"It's going to be another war next week with the quality they've got on show. We've been in this position before and we know what's waiting for us," said Harlequins number eight Nick Easter.
Chris Robshaw may not be the best athlete in the world or the most dynamic carrier. But he is brave to a fault, goes in where it hurts most, and keeps motoring from first minute to last.
Leinster's leader Jamie Heaslip was hindered by what was happening in front of him. The scrum ball was messy and he had to work mostly off the back-foot.
They have to establish dominance in the return-leg or they will be beaten up again. It is as simple and brutal as that.
2. THE FRONT ROW
The simple fact that Leinster's back-up front row of Tadhg Furlong, Bryan Byrne and Michael Bent played a total of four minutes - Furlong replaced Ross in the 76th - tells you everything you need to know about the lack of confidence placed in them.
In fairness, O'Connor took the view that protection was a better option than exposure, not because Furlong and Byrne aren't able, but because they are not quite ready.
The Irish province won three out of five scrums, a dreadfully inept 60% return, while Harlequins were able to launch off seven out of eight resets there.
In reality, scrum coach Marco Caputo is already working without an all-Ireland international front three in Healy, Strauss and Moore.
Even when Mike Ross, Sean Cronin and Jack McGrath were coming under heavy fire from Joe Marler, Dave Ward and Will Collier, O'Connor and Caputo did not take a punt on either Byrne or Bent to rectify the matter and left Furlong's impact too late to matter.
It is most likely Leinster will hit the live scrum session hard this week. They have to turn a negative into a positive in double quick time.
3. CUTTING EDGE
Possession does not always translate into points. Leinster are beginning to look like a side with fewer problems when they don't have the ball against when they do have it.
In other words, they look more secure in defence than they do dangerous in attack.
Okay, Harlequins cut them for two tries. The first was born out of Leinster's inaccuracy. Sean Cronin's crooked throw coughed up a scrum.
Referee Jerome Garces whistled for a penalty there. This was turned into a five-metre lineout and 'Quins patience and power enabled Easter to hit the inside shoulder for a try that was all of Leinster's making.
The second from left wing Aseli Tikoirotuma came down to the bluff of stepping up for an interception with no one left at home to mind the house.
It could so easily have gone the other way if Rob Kearney had found Zane Kirchner close to the touchline.
4. ROTATION IS A KILLER
O'Connor said it himself last week.You can't play the rugby you want when "you are changing ten blokes every week."
On the balance of probability, Leinster should be better, much better at The Aviva Stadium on Saturday night.
They know that. Easter knows that too. "We have to be ready for the same threats we were mindful of. They are going to be a lot sharper," he said.
"I've been down there before and it's a very hard place to play, especially when they get on the front-foot. We'll be about stopping that."
Again, the importance of physicality comes into focus.
Penalties kill momentum. The magic number is anything in single digits, below ten. Leinster handed Harlequins twelve long-arm penalties, even though some of them were highly questionable.
They also conceded 16 turnovers, made four line breaks and beat 18 defenders without even going close to scoring a try. This is not good enough at this level.
Were it not for the immaculate place-kicking of Ian Madigan, Leinster could well have departed London without even a losing bonus-point.