Neville's colleague Jamie Carragher also voiced concern about the World Cup winner.
"I think a little bit of a worry for Manchester United fans will be Martinez because I was very critical of him when he first came," Carragher said. "I didn't think he'd be able to play in a back four because he'd be too small. But he had a really good first season.
"In some ways you think he's perfectly suited to a back three. He can step out with the left foot, got two bigger players alongside him. Even though he played a great ball through for the winner in the Manchester derby, I don't think he's looked as comfortable since Amorim came in."