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 teamed up with Crawshay's to tour Georgia in June. The side, which will be called the Crawshays WRU President's XV, will play Emerging Georgia twice, in a tour which supports the Welsh Government's efforts to grow relations between the two countries.
Wales Sevens are looking to re-group after a disappointing tournament last time out in South Africa. The next double leg in New Zealand and Las Vegas offers Gareth Williams a chance to redeem themselves.
Wales Women's head coach Rhys Edwards was at Llandovery to see the conclusion of an entertaining Women's Regional championship where any one of three teams were in contention for title glory on the last day of action.
Christmas has come early at Glynneath rugby club, one of a number around Wales who have received an array of merchandise and equipment this December courtesy of the Welsh Rugby Union's £750,000 participation points scheme.
The WRU have been delivering on the scheme - which gives individual clubs access to up to £5,000 worth of supplies and equipment each for the 2014/2015 season - over the last two weeks and today (Friday) were received by a special guest in Glynneath, club president and pitch marker Max Boyce.
As the countdown continues for RWC2015, Tournament Chief Executive Debbie Jevans visited the Millennium Stadium to discuss preparations for the eight matches and reveal the positive news that 100,000 tickets for games at the home of Welsh rugby have sold in the last seven days.