The 27-year-old, who joined the Black Cats on a six-month contract in January, failed to secure a regular spot at the Stadium of Light due to Vito Mannone taking the number one place.
"I'm in love with Spain," the Northern Echo quotes Ustari as saying. "I really like Spain, the people, the quality of life. The truth is I'd go back. I love the league too. I feel like I identified with the league, with how people live it. I have fond memories."