Leinster boss Joe Schmidt was left frustrated with his side's second loss to Clermont in a week despite grabbing a losing bonus point.
A late try from Fergus McFadden gave Leinster a slim chance of qualifying for the knockout stages, with the champions needing to take maximum points from their remaining Pool matches against the Scarlets and Exeter to defend their title.
Clermont were physically dominant, with Schmidt highlighting the strength in depth of their squad as being a key factor behind their victory.
"Miracles happen in sport, I guess, but mathematical long shots very seldom come into being," said Schmidt.
"I don't think we have the same depth as some of the big-spending French clubs.
"It's hard to take defeat two weeks in a row, with the amount of effort the players put in.
"The other challenge for champions is they always look pretty good when they are on top. It is how they come back from adversity that is the real challenge."