The Brazilian scored two and set up two as Madrid raced into a 6-1 lead before conceding two goals late on.
And the 29-year-old showed no ill-effects from his persistent injuries as he completed 90 minutes at Mestalla.
"This will be great for my confidence," the player said after the game.
"A team that produces a performance like that after winning the cup shows that we want to finish the season strongly - we are a fantastic group."
But Mourinho opted to focus on the Brazilian's individual contribution.
"He played an absolutely fantastic game," the Portuguese said.
"It's fantastic for his confidence and we now have six or seven games to play in which we will need Kaka at his best."