“I'm very excited," said the 37-year-old, who has lifted the World Cup with Spain and twice celebrated European Championship success.
“The opportunity came and it was good for both and so here we are.
“To help us both to grow and hopefully to achieve our target of staying up, and fight for every single ball from now on."
He added: “I'm still hungry and still humble enough to keep growing and keep learning.
“And at Villa, it was not because they offered me regular football, I have to earn that, convince the manager and be ready to rumble straightaway.
“Mentally, I'm ready for this challenge, physically, I'm absolutely grateful for being 37.
“Let's go, let's start, I can't wait."