In an exclusive interview with MUTV, the Reds boss admitted it was difficult to prise last season's Barclays Premier League top goalscorer from Arsenal but is confident the Dutchman will have a huge impact at Old Trafford.
"It wasn't an easy one," said Sir Alex. "Understandably, Arsene Wenger didn't want to sell to Manchester United - he's already mentioned that in the morning papers. The boy wanted to come to us and that's important. He turned down various clubs to join us because he wants the challenge of coming to the biggest club in the world. I think that's fantastic.
"That's what swung it and what made it possible. If he hadn't come out forcibly to Arsenal to tell them he wanted to go to Manchester United, it meant the negotiations were really over - the only negotiation left was agreeing a fee with Arsenal as there was no-one else in the picture because the boy wanted it and was desperate to come to Manchester United."
When asked if the opportunity to sign a player of van Persie's calibre was simply too good to resist, he replied: "You can't turn that down. I didn't think it was possible we could get van Persie when we talked about it last year but, when I heard he'd refused to sign a new contract and made it publicly known that he wanted to leave Arsenal, then we had to be interested - there's no question about that.
"I spoke to David Gill and David spoke to the Glazer family and we got the ball rolling but that was some months ago. It's been a long haul and it's not been an easy one but, thankfully, we've got him here."