United announced an agreement to sign the 18-year-old from Atalanta in the summer, adding Diallo would join the club in the January transfer window.
"If you see his clips and speak to some of the people at the club that were involved in buying him…. potentially this kid could be an absolute world beater.
"I think that's the thoughts of Manchester United people that they've unearthed…. I'm not going to give him that pressure of Cristiano Ronaldo but nobody knew about Ronaldo when we bought him really other than people in Portugal.
"This kid seems quite similar in that sense. if you see his clips and the way people talk about him then I think the world is his oyster if he applies himself right."