Manchester United have confirmed the signing of Marcos Rojo from Sporting Lisbon for 20 million euros, with Nani joining the Portuguese club on a season-long loan as part of the deal.
Argentina defender Rojo has signed a five-year contract after completing a medical on Wednesday.
The 24-year-old had apologised to Sporting after refusing to train while the Portuguese club resolved their dispute with management company Doyen Sports, who owned some of Rojo's economic rights, before a fee was agreed.
Portugal winger Nani, meanwhile, returns to the club he left for United as part of the deal.
Rojo told his new club's official website: "It is such an honour to say that I now play for Manchester United. The Premier League is the most exciting league in the world and to have the chance to play in it for the world's biggest club is a dream for me.
"I am young and am very keen to continue to learn the game, so playing for such an experienced technical coach as Louis van Gaal is a fantastic opportunity for me. I have joined United to work hard with my team-mates to win trophies and I know that the manager shares that ambition."
United boss Louis van Gaal added: "Marcos is a very gifted defender. He has played at the highest level in world football and can play either as a central defender or left-back. He has ability, physical strength and a willingness to learn that means he has a very bright future ahead of him.
"He had a very strong World Cup and has been playing in Europe for a couple of years now. He is a very good addition to the team."
United, who had only signed Luke Shaw and Ander Herrera so far this summer, turned to Rojo after accepting defeat in their attempts to sign Mats Hummels and Thomas Vermaelen.
They have been short of defenders after the summer departures of Rio Ferdinand, Nemanja Vidic and Patrice Evra and Van Gaal's decision to switch to a formation that requires three centre-backs.
Shaw is out for four weeks with a hamstring injury and Van Gaal had to give 20-year-old Tyler Blackett a debut in the middle of defence in Saturday's defeat to Swansea so Rojo is likely to come straight into the team for Sunday's game at Sunderland.