The former Manchester United winger started for Tuesday night's 1-0 victory.
Afterwards, Garnacho was asked about being frozen out at United over the summer.
"I think it was a difficult moment there, just training alone," Garnacho told TNT Sports. "But I have nothing bad to say about the club, my old club Manchester United.
"It was just a bad moment in life but now I’m very happy to be here and to play in this competition and to get the three points today."
Chelsea manager Enzo Maresca also said of Garnacho's performance: "Alejandro was very good today. Not only on the ball but defensively, he was very good in the way he pressed.
"In the last third he has freedom to go one-v-one when he decides. To arrive in the box is important because it’s from there we score. I was pleased with him."