Conor O'Shea struck a blow for The Premiership at the expense of his home province as Harlequins took over at the top of Pool Two.
"We're three-from-three in Europe. That's not a bad start. But, we face a massive task (in Dublin)," he said.
Beforehand, Leinster knew they could have killed the Pool almost stone dead by matching the result they took away from the Heineken Cup quarter-final, subsequently dubbed 'Bloodgate,' in 2009.
The winner would be rolling in clover until the return leg six days later; the loser would be stewing in anger and anxiety.
Leinster can have few complaints. They were beaten by six points and two tries to none.
"These double-headers are pretty hard. It's 4-1. Leinster will look at things they could have done to win. They'll go away and they'll have a point," added O'Shea.
There were periods when Leinster threatened to break out of the Quins stranglehold, only for mistakes to hold them back.
"It was just our reaction when the adversity came, when we were under the pump. There was a real steel. There was a real resolve. We talk about that." said O'Shea.
The English club, located right across the road from Twickenham, brought all the motivation in the world under the remit of ex-Terenure College and Ireland full-back O'Shea, the first Leinster player to dot down for a try in the Heineken Cup back in 1995.
"We'll just have to make sure we're ready for The Aviva and we'll have to ramp it up another 10%, 20%, 30%. I know the players will be buzzing after tonight.
"We know what's coming next week. They are a brilliant side," O'Shea said of Leinster. "We have to go over and steel ourselves, try and get a team on the pitch because there are a few bumps and bruises."
Their main source of worry must be around the groin injury picked up by out-half Nick Evans.
Harlequins forwards coach John Kingston, formerly of Connacht, was the man who kick-started Mike Ross's career by plucking him out of obscurity and turning him into a prop solid enough to attract Leinster.
And it was at the scrum where Harlequins exerted their authority, with Joe Marler getting up in under Ross and Will Collier troubling Jack McGrath at times.
"Luckily, we had the upper-hand in that area," said Harlequins number eight Nick Easter.
"We weren't happy with the way the scrum went there (last week), took the lessons from that and it can happen very quickly in a week.
"The way the scrum is going at the moment this season, it's a real contest and, from one week to the next, it is very hard to predict how it's going to go."