The 30-year-old, who has recently returned from a long-term leg injury, scored in the Blaugrana's 5-0 victory in Madrid.
"I'm in great shape, and I am very happy to return to playing football again," he told AS. "I am delighted at helping my team win by getting a goal.
"It so happens that I have never played at this stadium [of Rayo]. I am happy to debut here, score and win and that everything turned out fine."
Barca remain at the top of the league standings with 25 points from nine matches.