Darmian left United this season for Parma.
He told the Guardian: "I still remember Marcus Rashford's first session with the senior squad like it was yesterday," he said.
“I immediately thought: 'wow, this is an incredible player'. He did so many impressive things for a player of his age and the way he improved in every detail really impressed me.
“If he continues to work the way he has done so far he can be a Ballon d'Or winner.
“I'm convinced of this. He impressed me the most of all the players at United and he's very young and can still improve.
“For me he can reach the level of Kylian Mbappe and compete for the Ballon d'Or for years to come."