Hartson wrote in the Scottish Sun: "David Moyes isnâ€™t up to the job of being Manchester United boss.
"There, Iâ€™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.
"Donâ€™t get me wrong, Iâ€™ve nothing against the man. Moyes probably got the best out of what he had at Everton.
"But I donâ€™t see him as this management messiah everyone is making him out to be. Iâ€™ve got to be honest and for me heâ€™s not capable of being in charge at Old Trafford.
"Theyâ€™ll regret making this appointment if you ask me."