Big Ron believes they're lacking a midfield "destroyer".
"When you look at them through the years, they've had a Bryan Robson, a Paul Ince or a Roy Keane," Atkinson, who was in charge of United between 1981 and 1986, told BBC Radio Manchester.
"I think Fergie would love somebody like that to play in there now. Great players, but someone who will frighten the opposition a little bit when they've got the ball.
"There's been a lot of criticism of the back four and maybe sometimes it's because they haven't got a midfield destroyer in front of them that puts the back four under more pressure."