Merson wrote for the Daily Star: "United are on the verge of a meltdown - because that's what it would be if they don't finish in the top four.
"They were top of the league not so long ago and if you're in with a chance of winning the title and then you don't finish in the top four, that's a major collapse.
"United have stopped scoring goals and teams below them have winnable games. They have opened the door. And they could be caught.
"You have to ask questions of the players. Look at the players they have. Edinson Cavani, Marcus Rashford, Mason Greenwood, Daniel James.
"There's a lot of ammunition there. Players who can hurt you. But too often it's all about Bruno Fernandes.
"They're always waiting for him to do something special. But teams are targeting him now and he's getting tired. It's taking a toll on him."