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.
The Webb Ellis Cup visited Lampeter on Monday to recognise the fact that the town is celebrating a major milestone as it is believed the first ever game of rugby played in Wales, was in fact played in Lampeter 150 years ago.
Cardiff Blues U18 Girls will go into Sunday's Gemau Cymru with an extra spring in their step after Welsh internationals Elinor Snowsill and Philippa Tuttiett offered some last minute tips at a training session earlier this week.
Rob Howley and Josh Lewsey addressed more than 250 community club coaches and referees at the first WRU National Community Conference on the weekend. The volunteers benefitted from coaching masterclasses and workshops in a bid to connect all levels of the rugby pathway in Wales.
UEFA has today confirmed the Football Association of Wales (FAW) has been successful in its bid for the Millennium Stadium to host the 2017 UEFA Champions League Final on Saturday 3rd June, 2017 (kick-off: 19.45hrs GMT).