United have suffered three defeats in their opening eight games under David Moyes and Hughes warned that Sir Alex Ferguson’s successor cannot afford many more if he wants to retain the title.
"You have to be careful," he said. "There are only a certain amount of games you can lose; five or six enables you to still be in the title race. Lose any more than six then you make it very difficult for yourself, nigh on impossible.
"They have lost three at the moment. They can't afford any more slip-ups. Maybe they are a little bit vulnerable at the moment. Invariably when people question Manchester United they come back with the answers."
Asked if he had sympathy for Moyes, Hughes said: "He probably expected it to come at some point. That was always going to be the case. He is stepping into the shoes of a great footballing man. It was always going to be difficult.
"There will always be difficult periods in a season even if you are manager of Manchester United, who more often than not win football matches."
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