Nigel Davies admitted Simon Easterby's yellow card cost his troops a valuable victory following the slender defeat against Glasgow Warriors.
The Scarlets trailed 13-11 when experienced flanker Easterby was harshly sin-binned for an altercation with Glasgow skipper Alastair Kellock in the 67th minute.
But Davies reckons he will have to read the riot act to his players in a bid to stop the ill-discipline reoccurring. He said: "I'm really annoyed because we deserved to pick up something from the game.
"We bombed a couple of guilt-edged opportunities in the second half but our discipline has become a huge issue again.
"We've made a big point of it but in our last two competitive games we have been penalised as a result.
"Simon's sin-bin was at a key stage of the game and the kick allowed Glasgow to put us out of reach of the bonus point.
"Simon has told me he was yellow-carded for retaliating and you can't afford to do that. It's a difficulty for him because he is a player and a coach and as coaches, we're adamant that our discipline will get better.
"There's a responsibility on all the players whether they are young or old. But we will have to sit down and talk about that because we can't play without 15 men on the field."
The Scarlets rallied in emphatic style after Rob Dewey's first half try at Firhill put the hosts in command. Full-back Dan Evans crossed before the break and Rhys Priestland kept the visitors in touch before Easterby was ordered off by Irish official John Lacey.
But the Scarlets chief insists there were positives to take following his side's first defeat of the season. "I'm hugely frustrated because it was a game we could and should have won. We didn't control the game in the first half but we showed plenty of endeavour and character and the line-out was a strong point for us. But we're just not helping ourselves."
Rhys Thomas, who was axed following his yellow-card last weekend, is expected to return for the Scarlets' home clash with Munster next Saturday.
Nearly 40 players were involved in the first Wales Under 18 training session of the season. WRU National Performance Manager Gethin Watts is excited at the potential shown by the group and expects the Regional Under 18 Championship to hone their skills even further.