Simon Easterby was left to rue Morgan Stoddart's red card after Scarlets were crushed 49-16 by Clermont Auvergne.
The Wales wing was dismissed after picking up two first-half yellows at Stade Marcel Michelin and Clermont took full advantage.
Easterby, whose side led 7-0 after four minutes thanks to Jon Davies' converted try, said: "We came with a good game plan and it was going well for 30 minutes, but the sending off changed everything.
"The red card is harsh to say the least. Certain things were going our way, we were sticking to our plan but they scored a soft try with virtually their first attack of the half.
"I didn't see the first sin-bin, the second if Morgan didn't roll away should it have been a penalty try?
"But we didn't get the defence right on Morgan's wing and they scored three tries."
The Scarlets welcome reigning champions Leinster, who they beat 45-20 earlier this year, in the next round of matches and having been drawn in what has been billed a 'group of death', Easterby admits that game is make or break.
He added: "We will take a lot from this game, we took a bit of a beating which will be tough to take. The players in there to a man are bitterly disappointed, but this group is still very much alive and we are very much alive in it.
"But next week is a must-win game. If you lose your first in Europe, you have to win your second."
Meanwhile, Clermont travel to Exeter Chiefs, who were narrowly beaten by Leinster in round one, full of confidence after surviving the early scare and running away with the encounter.
Head coach Vern Cotter said: "To come away with a bonus-point was pleasing after the way the game started. They had us in a lot of difficulty, we probably didn't get certain things right and had to claw our way back into it.
"But to get five points is a great start but we know how tough it will be in Exeter next week."