Leo Cullen's side - who have made a strong start to the GUINNESS PRO12 with five wins from their first seven games - are still working to reintegrate their Irish World Cup players. But Sexton insists that is no excuse.
"It wasn't a great day. For most of us it was probably the worst day we've had in a Leinster jersey," he said.
"It's very disappointing to do that at home, in front of our supporters, in the first game of the European Cup.
"It's one thing losing at home, another thing losing by the margin that we did."
Next up for Leinster is a trip to the Rec to take on Bath - a fixture that has rapidly become a must-win game for the Irish province.
And while Leinster are sweating on the fitness of key players like captain Isa Nacewa and flanker Sean O'Brien, Sexton is confident his side can turn it around.
"We've got to move on now. It's a short turnaround to Bath and already our championship hangs on this game and we've got to show a reaction," he added.
"We've got to be better with the ball, we've got to show everything we didn't show this week.
"We're trying to work in a new structure at the moment and it's a good structure to play in, it allows you to get on the ball and make decisions but there are parts of that structure that broke down badly at the weekend and that need to be a hell of a lot better.
"There is parts of our structural play that need to improve but other parts like winning 50-50s and winning collisions, I suppose they are the parts that are unacceptable that you can't turn up to a rugby match and not win them."
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