Scarlets captain Ken Owens admitted his side contributed to their own downfall after they had three players sent to the sin during their derby defeat to the Ospreys.
Rob Evans, Will Boyde and Steff Evans were all given yellow cards at the Liberty Stadium as the Ospreys came from behind to win 19-9 and move to second in the Guinness Pro 12 table.
Neither side crossed the line in a dour game low on excitement, but the Ospreys did manage a penalty try, with Dan Biggar also landing four penalties and a conversion.
Wales hooker Owens said: “You cannot win games like that against a quality side like the Ospreys when you are down to 14 men for the majority of the second-half.
“They put us under pressure and we didn’t get it right. We got guys on the wrong side and to be fair to the ref, he has to make decisions early on. We wanted a quick game, so we have to respect that.
“I have been coming here for a decade now and I don’t think I’ve won a Boxing Day match. It’s bit frustrating.
“All credit to the Ospreys, they beat us in the contact area and taught us a lesson there. We put a shift in, but they beat us in that key area.”
The Scarlets’ scores came with three first-half penalties from Rhys Patchell, but he was over-shadowed by Biggar who became the first player to go past 2,000 points in regional rugby.
Ospreys skipper Alun Wyn Jones said: “The Scarlets shut us down in the first-half and starved us of possession.
“But the message at half-time was to keep chipping away and keep the ball. The penalties came and we got on the scoreboard.
“We haven’t been under that much pressure at home very often this season, so this was a good test going into 2017.”
It was a cagey opening 40 minutes with the visitors enjoying a 9-6 interval lead through three Patchell penalties.
Defences appeared to be on top as the both sides gave away penalties during their opening incursions into opposition territory. And the absorbing stalemate was not broken until Biggar kicked a 15th-minute penalty after the Scarlets transgressed at a scrum.
There was a blow for the away side when two of their more influential players Scott Williams and Aaron Shingler had to leave the field for treatment after a clash of heads. Williams returned but Shingler was unable to continue following an assessment.
The Scarlets suffered a potentially more worrying setback when Rob Evans was sent to the sin-bin for a high tackle on wing Dan Evans.
But the Ospreys could not take advantage of the extra man with Wyn Jones being held up over the line following good work from flanker Boyde.
The Ospreys earned a penalty on the 22 which Biggar looked sure to kick but the ball sailed the wrong side of the right upright and the hosts were punished for the miss with Patchell kicking penalties after 34 and 39 minutes as the Scarlets back-row continued to boss the breakdown.
But the Ospreys had the last say of a nip-and-tuck first half with Biggar kicking a final-minute penalty from close to halfway.
The key moment of the second half came after 54 minutes when the Ospreys were awarded a penalty try with Boyde penalised for dragging down a maul for which he received a yellow card. Biggar converted and then kicked a penalty six minutes later.
Any chance the Scarlets had of getting back into the game and even securing a losing bonus point were dashed when Steff Evans was sin-binned for dragging down a maul after 70 minutes.
Two minutes later Biggar kicked his fourth penalty as the Ospreys secured their 11th straight home win in all competitions this season.
