The skipper and his side were slayed 37-5 in the Champions Cup quarter-final clash, leaving their European hopes in tatters.
Clermont flew into a 27-0 lead and never looked back on their way to a comprehensive four-try victory.
And Hartley was pulling no punches in his assessment of the game, knowing clinical Clermont deserved to book a semi-final spot.
“Early on we went 10-0 down and we had a chance to put three on the board but missed,” said the Saints captain. “Then we had two opportunities in the 22 and came away with nothing and the lead just got away from us.
“They kept building and building and we compounded error on error. We were steamrollered.
“They got everything right: emotionally they were right, the intensity was there for 80 minutes and they took the opportunities every time we gave them one.”