The Top 14 outfit had not been beaten on home soil in Europe's top-tier competition but tasted their first defeat at the Stade Mayol on Saturday.
England No.10 Farrell was making his first appearance of the season after recovering from a back injury and looked like he had never been away.
The 25-year-old was delighted to be back on the pitch and feels Saracens can get even better, despite what was a historic victory across the Channel.
"It was a great effort to come to a place like this and get a result is a massive scalp," he said.
"But there are still things to work on, our discipline was not great in the second half and I am sure we will look at that.
"I was a bit stiff coming towards the end of the game. I was ready for that, I was itching to be back out there."