United formally signed the 18-year-old from Crewe on July 1, but had put the deal in place some time earlier fearing a dazzling display in the League Two play-off final at Wembley might prompt other suitors to join to fight to sign him.
Ferguson told www.manutd.com: "We thought that if we waited until the play-offs, the secret would be out, so to speak. All the TV cameras would be focussed on him and it might alert a few other clubs."
He added: "Mind you, there were already quite a few interested. As well as Arsenal and Chelsea, we knew Manchester City were keeping tabs on him as well, so I phoned [Crewe director of football] Dario Gradi, told him we were keen and we started negotiating.
"We sorted the deal with the chairman and agreed we'd leave it until after the play-off finals.
"It was all in place a good 10 days beforehand. Nick's agent told Nick after the match and then he came up to my house the next day and we got the deal done."