They reached the quarter-finals seven times in the early years of the competition and were three-time semi-finalists in 2000, 2002 and 2007.
But their hopes of automatic qualification for next year's tournament were dashed after losing to the Blues in the Magners League.
Now they must swallow their pride and cheer Cardiff to lift the Amlin Challenge Cup and earn an extra place for Wales by lifting the cup.
Scarlets boss Nigel Davies said: "Unfortunately that is the position we find ourselves in now. We felt we were good enough to win our remaining games but now we need the Blues to do us a favour or we're playing in the Amlin Challenge Cup. The quality in the pool stage of that competition just isn't the same as Heineken Cup rugby."
Cardiff coach Dai Young admitted his team would fly the flag as the last Welsh team in Europe against England's final hope, Wasps.
Young said: "The pressure is off us a little now after qualifying for Europe. We want to win the Challenge Cup for ourselves, but if we can go all the way and help the Scarlets at the same time, it would be an added bonus."