The Dutchman is now on 24 Premier league strikes for the season after his 32-minute hat trick against Paul Lambert's side at Old Trafford
After opening the scoring in the second minute with a close-range strike, Van Persie received a raking pass from Wayne Rooney before firing first time past Brad Guzan. The 29-year-old completed his hat trick with another finish inside the box 12 minutes before halftime.
"It was goal of the century for me," Sir Alex told reporters.
"It was marvellous. The technique with his head down, over the ball and just perfect timing. It was a magnificent strike. He scored exactly the same goal against Arsenal. He is in his mature years and winning the league tonight meant a lot to him.
"He's been unbelievable. I think that he has to take a lot of the credit. I think his goals tell you that, his performance levels told you that. But I'm sure Robin would be saying what a great bunch of players he's had with him."
Van Persie is now one goal clear of Liverpool's Luis Suarez in the race for the Premier League's Golden Boot, while Sir Alex has compared the striker's impact to Eric Cantona's time at Old Trafford.
"When you identify a great player coming to the end of his contract, you go for him no matter who," he added.
"In terms of impact he has made as big an impact as any player I can imagine. [Eric] Cantona was an incredible impact player, I've had some great strikers, maybe 10 great strikers.
"We had an expectation of him. He was sensational last year, but Arsene [Wenger] said he was better than I'd think after I'd done the deal, and he was right. He has never disappointed us."