The Dutchman had gone 10 matches without scoring before slamming home a penalty mid-way through the second half for his 20th goal of the campaign, to add to midfielder Michael Carrick's fourth minute opener.
United now need only seven more points from their remaining six matches to clinch their 20th championship, and require second-place City to win all seven of their remaining matches to stand any chance.
"You go through spells when you don't score where you never think you're ever going to get a goal, and then when you are scoring you think you're never going to miss," Ferguson told Sky Sports.
"Today was an important goal.
"He did really good work from Wayne's pass to get brought down for the penalty.
"The ball moved off the spot two or three times, the composure to settle himself down and the penalty kick was a perfect penalty. Low hard, straight into the corner, great penalty."
And after Van Persie jumped on his manager during the celebrations, he added: "He nearly killed me. He forgets I'm 71.
"But that was a great celebration because it brought the emotions of the fans and the players together, everybody could celebrate that."