The Saints were demolished by the three-time winners of the competition, as Luke Fitzgerald registered a hat-trick of tries, with Jamie Heaslip, Eoin Reddan and Brian O'Driscoll also getting in on the act to consign the 2000 winners to a record-equalling heaviest defeat in Europe.
"We were embarrassed out there and came second best. We made a terrible start and that didn't improve," he told reporters. "Sometimes you make bad starts and have a bad five or 10 minutes, but you at least expect to come back fighting. But we were completely outclassed.
"I don't know why that fight was missing. It was a game we were building up to, it was our big game - at home against one of Europe's finest teams.
"I feel embarrassed because we've let the fans down. We have some great fans here. They love their European rugby. It's a great competition for us but we've messed up."
Leinster remain top of Pool 1 in Europe's elite tournament, five points clear of Castres.