The Red Devils attempted to sign the England international in 1988 when he was at Newcastle but the midfielder would eventually join Tottenham.
Ferguson told SportBible: "I think he was the best English player since Bobby Charlton. I think he was a fantastic player.
"Unfortunately, we didn't get him and I think, looking back now, he made a big mistake. He recognised it himself years later. But we had Geordies in the camp. We had Bobby Charlton, Bryan Robson, Steve Bruce, even Gary Pallister from Middlesbrough.
"We had people there who would have taken care of him, particularly Bryan Robson who is fantastic with players.
"Instead, he goes to an island in London, and you're easily swallowed up there as a young lad.
"I think that was a big mistake by him. He was a marvellous player. I loved his strength.
"We played Newcastle up there and he had just come back from injury. As soon as he came back, they started winning games. I played three in the middle of the pitch – Whiteside, Robson and Remi Moses and he destroyed them.
"He nutmegged Moses right in front of the dugout and patted him on the head.
"He was unbelievable and I said to [former Man Utd chairman] Martin Edwards on the way home: 'Sign this boy. Get on the phone to him on Monday.'
"And by that time, he had already done a deal with Tottenham, which was really unfortunate. That was a disappointment because he agreed to sign and then Tottenham changed his mind by buying his mother and father a house."
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