The Spanish fullback, who signed for City last year from Girona, has spent this season on-loan with Valladolid.
City are now expected to place him at Sporting CP for next term and Porro said his farewell to Valladolid last night.
He posted to social media: "Thank you, Valladolid. I will always carry this club in my heart and remember the fans who treated me with great affection and respect in a difficult season.
"Thanks to my teammates, coaches, doctors, assistants, staff and management ... Until always."