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In a further blow for the Scottish side they were also stripped of the services of veteran Gregor Townsend after he was forced from the field with an ankle injury after only ten minutes.
It was a nightmare return for comeback hero Townsend after he was caught in a pile of players while helping to set up the opening try of the game.
Forward Kelly Brown was able to collect for the score but there was no time for celebrations as Townsend’s return was brought to a premature end.
Reivers began well with a penalty from Charlie Hore but the lead was short-lived as American Mike Hercus slotted the equaliser moments later.
Callum MacRae came on for the stricken Townsend with Hore switching to outside half and despite being reduced to 14 men it was Llanelli who seized the initiative in the build-up to the interval.
Dave Duley, who had come on as a blood substitute, was yellow carded for a rucking infringement but his misfortune appeared to fire up the visitors.
The result was a try for prop John Davies after he was driven over by his teammates.
Hercus was unable to add the extras but made amends with another penalty shortly after and their lead was stretched even further with a monster penalty from Barry Davies before the break.
The Llanelli pack took control of proceedings in the second period although Borders’ wing Simon Danielli came close with one strong break.
The Scarlets were once again down to 14-men when Iestyn Thomas was sent to the sin-bin for illegally preventing a forward drive but the Reivers were unable to take advantage.
The Scarlets appeared to make sure of the win when Hercus chipped over an opportunist drop goal but a flurry of points was still to come.
First Aisea Havili claimed a try for the Scarlets, converted by Hercus, before Ben MacDougall grabbed a consolation try for the Reivers that was converted by Hore.