Hartson wrote in the Scottish Sun: "David Moyes isn’t up to the job of being Manchester United boss.
"There, I’ve said it. Someone had to.
"That great club needs a great manager. Not an ordinary one. They need someone steeped in success. Not someone knee-deep in mediocrity.
"Don’t get me wrong, I’ve nothing against the man. Moyes probably got the best out of what he had at Everton.
"But I don’t see him as this management messiah everyone is making him out to be. I’ve got to be honest and for me he’s not capable of being in charge at Old Trafford.
"They’ll regret making this appointment if you ask me."