Ferguson is delighted to have got a deal done, with Powell tipped to be a top-flight star of the future.
He told United's official website: "We thought that if we waited until the play-offs, the secret would be out, so to speak. All the TV cameras would be focused on him and it might alert a few other clubs.
"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."