Jim Mallinder offered no consolation to Dylan Hartley after the England captain lasted just six minutes off the bench as Northampton were dominated 37-10 by Leinster at Franklin's Gardens.
Leinster had started in efficient fashion, Rob Kearney's fine line-break setting up young Ireland centre Garry Ringrose to go over after just two minutes.
When Isa Nacewa converted to put them 10-0 ahead after 11 minutes, it looked to be a very long evening for Northampton.
But Saints, to their credit, dug in - with Tom Wood in particular grafting magnificently in defence - and when Ahsee Tuala touched down in the 45th minute for a converted try, Northampton had every chance at 10-10.
Hartley's introduction brought warm cheers, but Sean O'Brien's try on 56 minutes and then the England hooker's departure two minutes later ended the contest and left Mallinder with no choice but to condemn Eddie Jones' Test skipper.
"You can't strike to the head. We've seen enough of them over the last few weeks and I think it was a red card," he said.
"Dylan has been playing really well and leading England by example, so it was disappointing on returning from England and doing such a good job that that happens.
"I wouldn't say it cost us the game. I think we started badly, came back into the game pretty well and we had just conceded a try, actually, so we were already a few points down.
"But I think when you go down to 14 men against such a quality side as Leinster then you're always going to be up against it."
After Hartley's dismissal, Leinster ruthlessly exploited the extra man. First, Ross Byrne's pinpoint cross-field kick picked out Rory O'Loughlin on the left wing for a comfortable finish.
Though Nacewa couldn't convert, the visitors claimed the bonus point with five minutes left as Jamison Gibson-Park scored a brilliant individual try down the left wing.
Northampton finished the match with 13 men after George Pisi was sin-binned for a dangerous tackle on Nacewa, who scored the hosts' final try to leave Leo Cullen satisfied.
"Obviously once the red card happens I thought we were pretty clinical in the way that we played in terms of managing the game in the right areas and just taking the opportunities when they came," the former Leinster captain said.
"It was pleasing how we closed the game out, but there were probably areas where we can be better at, so I'll have a look over the weekend and try to make a few changes when we come in on Monday.
"The score was 20-10 at the time and I thought the guys showed good composure again - Rory (O'Loughlin) did unbelievably well from Ross (Byrne's) cross-field kick.
"That was really pleasing to see those new European caps come in, but the red card definitely knocked the stuffing out of them."
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