Wales centre Jonathan Davies played a solid 40 minutes on an early return from injury but it came in a losing cause as Scarlets were overpowered by hosts London Irish.
Davies, who suffered a pectoral tear playing for Wales against South Africa in November, was originally expected to be out until the end of February.
But, even though his outgunned side suffered a 29-10 defeat in the LV= Cup clash, he proved a handful for the Exiles as the main attacking force before being withdrawn at half-time.
Andrew Fenby, Shane Geraghty, Jamie Hagan, Sailosi Tagicakibau and Myles Dorrian ran in tries for Irish, with Geraghty and Ian Humphreys landing a conversion apiece.
Scarlets replied with a penalty from fly-half Josh Lewis and a try from George Earl, converted by Aled Thomas.
James O'Connor, the out-of-favour Australia full-back who is Toulon-bound at the end of the season, was a late withdrawal from the Exiles side due to a tight hamstring.
Topsy Ojo took over the number 15 shirt, with Dorrian joining the replacements.
With neither side able to qualify for the knockout stages, Geraghty kicked the first two Exiles penalties into touch in the corner.
Referee Steve Lee played the advantage after a third Scarlets infringement, and Geraghty and Ojo combined to send Fenby in for the opening try in the fourth minute.
Lewis opened the Scarlets account with a penalty three minutes later.
South African prop CJ Van der Linde's first start for Irish lasted only 20 minutes before he was helped off with a leg injury.
Geraghty scored the second Irish try, working a one-two with centre Fergus Mulchrone to split the Welsh region's defence before adding the conversion.
It took three opponents to stop Davies in a strong run down the left but the visitors failed to make the pressure tell.
Irish responded with their third try. Hooker David Paice was stopped just short before the Exiles turned the ball back for Ojo to send Van der Linde's replacement Hagan crashing over.
Davies, working his way into the game, figured twice more in Scarlets forays before Irish struck again in the final move of the first half.
Former Ireland scrum-half Tomas O'Leary made his side's fourth try out of nothing, spotting Tagicakibau unmarked on the left and launching a a huge pinpoint kick into the arms of the winger who was left with a clear run to the line.
Irish, leading 22-3 at the interval, made five substitutions for the start of the second half as director of rugby Brian Smith sought to safeguard a number of his key players ahead of next Saturday's Aviva Premiership visit to leaders Saracens.
An injury to second-half replacement prop Jacobie Adriaanse reduced the visitors, who had used all their substitutions, to 14 players and meant that, after 57 minutes, the game went to uncontested scrums.
Scarlets, a man short, started to make more impact than they had with the full 15, but the numbers were levelled in the 69th minute when Irish centre Eamonn Sheridan was yellow-carded for taking out Scarlets winger Aaron Warren before he received a pass.
Scarlets responded with a late try from replacement lock Earl, converted by Thomas, but Irish had the final say with Dorrian scoring their fifth try and Humphries converting.