Argentine defender Santiago Vergini has rejoined Sunderland on loan from Buenos Aires club Estudiantes.
Vergini made 16 appearances for the Black Cats last season, also on loan, as they escaped relegation.
"I'm very happy to be here once again," Vergini, 26, told the club's website. "The challenge for me is to become part of this team and offer all I can."
Meanwhile, Sunderland midfielder El-Hadji Ba has joined French Ligue 1 club Bastia on a season-long loan.
The 21-year-old struggled to break into Gus Poyet's side last season and was restricted to just three first-team appearances.