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.
Beating Merthyr has boosted Carmarthen Quins hopes of reaching a top eight spot in the Principality Premiership. Quins captain Haydn Pugh is delighted with his team's progress and is determined that everyone from the Under 6s through to the senior team, enjoys the ride.
When the Wales squad were asked to nominate individuals who played a big part in their early playing days, Wales and Dragons full back and wing Hallam Amos thought of his former Monmouth School rugby coach John Bevan.
Sam Cross and Luke Treharne have been named as co-captains of the Wales Sevens side as head coach Gareth Williams names his 12-man squad to play in the first leg of the HSBC World Series in Dubai next weekend.
Bedwas head coach Steve Law and players Nicky Griffiths and Alun Rees extol the virtues of the close knit club as they prepare to host third place RGC 1404 - who are one place head of them on the Principality Premiership points table - on Saturday.
Many of the Wales squad have said Diolch (thank you) to someone who played an important part in their early playing days by nominating them to be invited to the Wales v South Africa match. One person, Gowerton Comprehensive geography teacher Mr Dean Mason was nominated by two of his former pupils, Dan Biggar and Sam Davies.
The second WRU disability rugby pilot has been delivered, this time in Caerphilly schools, culminating in a festival at the Caerphilly Centre of Sporting Excellence, Ystrad Mynach tomorrow. Plans are in place to set up a Disability Rugby Club in Newbridge to provide long-term playing opportunities.
WRU TV: Hear from inside the Wales camp as Neil Jenkins discusses this weekend's clash with South Africa and how the squad are looking to put in a strong performance to round of the 2016 Under Armour Series