Jim Mallinder stood by his decision to make wholesale changes to the Saints team after their crushing 60-13 defeat in Dublin.
Mallinder left a host of international players out, making 10 alterations to his side after the previous week's 37-10 Champions Cup defeat to Leinster.
And they were to suffer an even bigger blow at the Aviva Stadium, losing 60-13 in what was Saints' biggest defeat since Mallinder took over in 2007.
It was also their most points the club had conceded in Europe in its history.
And Mallinder said: "It's very disappointing.
"We knew it was going to be a challenge coming out here.
"We made changes to the team, brought some fresh faces in, we know that Leinster are playing well, we saw that last week, they're a quality side, full of international players who are very confident at the moment, so it was always going to be an uphill battle.
"I was pretty pleased with our start. We arrived in good spirits, were in the game, winning at one stage (Saints were 13-5 up after 21 minutes), which was pleasing, but a couple of injuries to two of our senior players, George (Pisi) and Calum Clark, really didn't help at all.
"We saw the quality of Leinster, particularly early in the second half and they performed in the way we aspire to."
On his decision to make so many changes, Mallinder said: "It's fairly obvious that we were going to be struggling to qualify for the next round so it was sensible to make some changes.
"It was good experience for some of the younger lads so it's all a squad effort, but the reality is that we've got three massive league games coming up over the Christmas period.
"We could have put our first team in, but those lads had got a few knocks last week and I think it was the right decision.
"We'll have to see what happens over the next few weeks."
And he added: "I don't think it was a B team.
"We rested some players who have played a lot.
"Some of our international players were playing for England against Australia when Ireland weren't two weeks ago.
"Some of our players haven't had the benefit of a week off so I wouldn't call that a B team.
"If you look at some of the players who were playing, there were quite a few of our first-team regulars."
Leinster ran in nine tries as they took control of Pool 4, in which Saints sit bottom on four points after as many games.
And when asked for his message to the Saints fans who had paid hundreds of pounds to watch the team, Mallinder said: "I hope the Saints fans have enjoyed their weekend.
"I hope they thought the Saints lads put their bodies on the line and tried their very best.
"Clearly we weren't the better team today, but I certainly don't think it was for a lack of effort."
Saints now face Sale in the Aviva Premiership at Franklin's Gardens on Friday night.
And when asked how much pressure was now on that match, Mallinder said: "They're all big games.
"We've not been performing well this season and we need to win that game next week."