He will spend the season with Blues sister club in Spain, the loan running until 30 June, 2021.
Medina has been involved in all Blues matchday squads this season and was an unused substitute in the last two Championship fixtures.
But with Head Coach Aitor Karanka well stocked for midfielders the 26-year-old has been allowed to rejoin his former side.
Blues brought in Medina from Cornella ahead of the 2019/20 season and was loaned back in the January, having made one appearance.
He was instrumental in helping Cornella to the Segunda Division B Play-Off Final, which they lost in July.