Manchester United have bolstered their goalkeeping options with the signing of Argentina international Sergio Romero on a free transfer. The 28-year-old former Sampdoria keeper worked with United boss Louis van Gaal at AZ Alkmaar and will provide competition for Spanish stopper David de Gea, should the latter remain at Old Trafford. Victor Valdes is set to be sold after refusing to play in the reserves last season while Sam Johnstone represents another option between the sticks. Romero told United's official website: "I am absolutely delighted to join Manchester United. To play for the biggest club in the world is a dream come true for me. Louis van Gaal is a fantastic manager and I cannot wait to get started on this new and exciting challenge in my career." ------- Paris Saint-Germain defender David Luiz has hailed Angel Di Maria as an "amazing player" that can help the club win the Champions League. Di Maria has not joined up with his Manchester United team-mates in the United States as he prepares to complete a move to the French champions, for a reported £46.5million. Louis van Gaal has suggested the Argentina winger is exactly the sort of player United need to compete in the Champions League, but instead the 27-year-old can help PSG realise their own European ambitions, according to Luiz. "For us it is always very good if we can sign a fantastic player like Di Maria," the former Chelsea defender. "I love him, he is my friend. I played with him at Benfica, when we were young.