"I have enjoyed my time at Benfica enormously and I have learned a lot there. But I'm looking forward to playing in the Premier League at Old Trafford and for Jose Mourinho. I'm keen to get started and make my contribution to the team's efforts to win more trophies," added the centre-back, who made 47 appearances as Benfica won a Portuguese domestic double last season.
"Victor is a very talented young player, who has a great future ahead of him at United. Our season last year showed us that we need options and quality to add depth to the squad and Victor is the first to join us this summer," said Manchester United manager Jose Mourinho.