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 WRU has launched a campaign to create a long term legacy for Welsh club rugby by highlighting the advantages of volunteering. Rhian Edwards, a volunteer at Seven Sisters, has enjoyed many benefits of her volunteering at a grassroots rugby club including being part of the Rugby World Cup volunteer workforce after being nominated by the WRU - and the WRU is asking for more people to develop their 'Welsh rugby roots'.
Wales head coach Warren Gatland and captain Sam Warburton say their first prioirty on Sunday in the opening game of the World Cup is to win against Uruguay and not get overly concerned with points difference.
With only days to go until the biggest rugby competition in the world begins, rugby fans visiting Cardiff this autumn will be able to enjoy a special display at National Museum Cardiff as part of the celebrations surrounding the Rugby World Cup.
Geraint John has taken up the new role of WRU elite coach development manager having arrived in Wales last week. WRU TV caught up with him at the WRU rugby department's strategy day at the Millennium Stadium.