Scarlets head coach Nigel Davies was left pondering what could have been after a failed Heineken Cup campaign.
The consolation for the Scarlets is an Amlin Challenge Cup quarter-final place thanks to a dramatic win in Castres.
Davies said: "It's great, we're delighted. It puts us into a play-off, but it's the what-ifs now as well.
"We were so close to doing something special this year, but we can still do it in the Amlin and that's what we'll look to do."
Sunday's results in the European competitions confirmed the Scarlets will travel to Brive in the last eight of the second tier Amlin Cup, a game which wil be played on Sunday, 8 April.
Davies added: "We missed a few opportunities and when we look back at the Heineken Cup that's probably been the difference.
"There's a huge amount of endeavour, there's a lot of quality coming through, a lot of the players are developing very well now and for that group of players to come here, to come to a side that's third in the French league and actually beat them will do them a fantastic amount of good."
Davies was particularly pleased with his side's ability to compete, particularly up front, despite the absence of 20 players through injury.
The Scarlets also had to contend with the loss of Rhys Priestland, playing at full-back, to a knee injury early in the second half.
Number eight Mat Gilbert, standing in for England squad member Ben Morgan, set the Scarlets on their way with an early try.
And flanker Aaron Shingler provided the game's highlight with a 90-metre dash to the line following a bold interception as Castres' skipper Chris Masoe seemed destined to score.
Davies added: "It's a fantastic result for us. Over 20 players at home [injured] and the boys came over here and put in a fantastic performance.
"This team we played are third in the French league, they were full-on and our boys went out there and competed and we came away with a result.
"There was a lot of information out there. Obviously we were getting the result from Northampton as well so that had a bearing on how things were going.
"And that kick - that last minute kick, although Steve [Stephen Jones] wasn't aware that that was an important kick, it ultimately was.
"Aaron was superb throughout in fairness to him.
"Somebody was going to score during that play and Aaron put in a great read and then for a back-row forward to run the length of the field and score a try like that was something special.
"You don't see that often."