Speaking to Safe Betting Sites UK, Stam insisted: “I always said it from the start, when he went to United, I said, 'They're probably going to be talking about his height'.
“It's a common thing in the Premier League that teams are looking for centre-backs that are strong and big because of the physical part of the game.
“Bringing him in, it's a challenge for him. It's going to be interesting how he is adapting.
“Already people are criticising him — and the coach a little as well about bringing him in — because of his height.
“He's struggling in the aerial battles. You don't need to be very tall to be a good centre-back, even in the Premier League, if you're clever enough, then you can handle a lot of situations.
“But you can see with set-pieces, which are becoming a very big thing in football, especially in the Premier League, you need to be able to fight in the air and to win your aerial battles.
“You see in the games against Brentford and Brighton, if the opposition is playing long balls, they're going to play them on him, because the majority of times he's going to lose it.
“If Ten Hag is keeping Martinez on the pitch, they need to find a solution to handle that.
“Otherwise, it's going to happen over and over again."