The Argentina defender joined the Red Devils from Sporting Lisbon on Wednesday and his new manager is convinced that he has a bright future in the game.
United agreed to a 20 million euro deal with Sporting Lisbon for Rojo on Monday that saw out-of-favor winger Nani go in the opposite direction on loan, with the club officially confirming the deal Wednesday.
MORE: Ronaldo ice bucket challenge | Pitch invader's free kick | WAGs
The 24-year-old becomes Van Gaal's third signing of the summer, following the arrivals of Ander Herrera and Luke Shaw, and the Dutchman is convinced he will be a big player for United in the coming years.
He told the club's official website: "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."