"It's a strike to the head".
"I think we've improved an bad lot, we've a stronger squad", O'Brien said before the squad departed for England.
Referee Jérôme Garcès showed no hesitation in handing the England captain his marching orders, and having only come on six minutes earlier, it was the type of display that will surely infuriate the head coach of the national side.
"Certainly, he's been playing really well and leading England by example so it was disappointing that on returning from England after doing such a job that that happens".
"A lot of the younger guys have come on an terrible lot and I think we're in a better place this time of year".
He added that the sending off came at a time when Saints couldn't afford to be weakened, saying: "We had just conceded a try actually, we were already a few points down".
Secondly, for much of the second half, Northampton played with 14 men after Dylan Hartley was shown a red card for a reckless hit on O'Brien.
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 risky tackle on Isa Nacewa, who scored the final try to leave Leinster coach 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.
Listen to live Champions Cup commentary this weekend: Wasps v Connacht, RTÉ Radio One's Sunday Sport, from 1300 on LW252 and DAB.