Zaha will join United formally at the end of this campaign.
He told The Sun: "Then in January, the approach came from United. I spoke to Sir Alex and he was in London with Sir Bobby Charlton.
"They were staying at a hotel in London while the United squad had already gone to Qatar. When I first met him, I was thinking it was crazy that Sir Alex was telling me he had been watching me for many years.
"I spoke to my family and knew it was the best thing to go up there and sign. The last thing Sir Alex said was to keep working hard and that is what I intend to do.
"Most people have wished me good luck. At the end of one game a ref said 'good game, Mr United!'"