Sir Alex told The Daily Mail: "No matter the years of experience we have of going for titles, last year was a different type of year. It's the first time anyone has beaten us on goal difference. That is what cost us the league and it won't happen again. Trust me on that.
"Robin is what we have needed for the last couple of years. We needed a finished player. We have had several players who have been this sort of catalyst, starting with Eric Cantona.
"We were still capable of winning the title without Robin but he gives us a sort of certainty for the future."
The United boss revealed that he had originally given up hope of signing the former Arsenal striker - but that hope was rekindled after Van Persie refused to put pen to paper on a new deal with the Gunners."I didn't think Arsenal would let him go, but then he refused a new contract and that's when we acted.
"It's been a long haul but we needed a finished player. Paul Scholes and Ryan Giggs are coming to the end and I need mature, top players to replace them."