Van Persie, who had failed to score in his previous 10 games, converted a penalty at the Britannia Stadium as United edged closer to the title with a 2-0 victory.
"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 News. "Today was an important goal.
"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."
The win has seen United regain a 15-point lead over rivals Manchester City in the Premier League table.