Warren was scheduled to be on the bench before Gareth Maule withdrew from the starting XV just moments before kick-off with an ankle injury.
But boss Nigel Davies' faith in Warren was fully vindicated as the 20-year-old delivered a blockbusting performance from the No.12 shirt which included tries either side of the halftime interval.
And Warren believes being thrust into the deep end at a moment's notice worked to his advantage.
He said: "I was asked to play just at the end of the warm up which was a huge shock.
"I was expecting to be on the bench but because the game was kicking off I did not have time to get nervous about playing.
"That meant I was just able to play my natural game and I felt really at home.
"I tried to get involved as much as possible and obviously I am delighted with the tries but the main thing was that the Scarlets won again.
"This season I was really looking just to develop as much as I could and any game time I got would have been a bonus.
"It is great working with Nigel and all the coaches at Scarlets who are so encouraging to young players. Everyone gets an opportunity and hopefully I have managed to take mine."
Part of the reason Warren was able to grab his chance was due to the absence of Scarlets' Welsh internationals.
However it was not so long ago that the likes of Jonathan Davies and George North were making their debuts in the famous red shirts.
And Warren says their rapid rise to the heights of world rugby is an inspiring example to the next generation of talent coming off the Parc y Scarlets' conveyor belt.
He added: "Wales more or less have a Scarlets' backline at the World Cup but that has given us the chance to prove ourselves in the RaboDirect PRO12.
"It is a great learning curve for me and the other young lads and we are just trying to soak up as much as possible.
"Jonathan Davies is a fantastic talent and a great guy to learn off. He has been excellent in this World Cup as has George North who has really come from nowhere.
"They have been given opportunities at Scarlets and Wales when they were young and have taken them which is what I want to do."
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