Cristiano Ronaldo, by comparison, has hit eight in four and has a total of 27 goals to Rooney's 10.
Rooney missed two gilt-edged chances in the 2-0 win over Portsmouth.
And Ferguson said: "He was working too hard on Wednesday night and I think that cost him the opportunity of getting goals.
"I wish every player had that same desire. It is a joy to see a young lad with that natural enthusiasm.
"Don't worry, he will be banging the goals in again soon."