The 24-year-old comes with the reputation of being a budding defender having starred for Ajax last season.
But he may have a difficult time if he lines up against Liverpool and Roberto Firmino is spearheading their attack.
The Argentinean recalls his battles with the Brazilian, calling him a ‘very difficult’ player to mark.
Speaking to Ole last year, he revealed: “In Europe I had to play against Firmino.
“And he is a very difficult striker to mark because he is constantly getting out of position.”
Martinez added: “That generates confusion, you lose the reference… If you go looking for him, he takes you out of your position a lot.
“In addition, he is a player who moves very well and, on his back, with the ball, it is very difficult to take it away from him. He is very good.”
Martinez won’t have long to wait to face Liverpool, with one of the biggest fixtures in English football slated for the third game of the new season on August 22.
He could well be in the starting XI after spending time together with Ten Hag last season.
On his latest signing, the Dutch boss hailed the player’s South American characteristics.
“He [Martínez] also brings a South American spirit, a controlled aggressiveness. I think it will fit really good to United and our way of playing,” he said of Martinez.
“He’s [Martínez] not the tallest but he is quite good in the air. I feel comfortable with that. Of course, you need the right balance. “He has good timing, that’s one of his capabilities. We bring him in to not strengthen the squad, but the team.”