Kagawa arrived at Old Trafford with a reputation for finding the net. He scored 12 goals for previous club Borussia Dortmund two seasons ago and topped that with 17 last term.
Ferguson said: "We have been low on goals from midfield in recent seasons.
"There was a time when Bryan Robson would give you a dozen goals and Paul Scholes in his heyday was good for ten.
"Lately we have not seen that kind of tally.
"I believe Kagawa will put that right which, with the addition of Robin van Persie, should ensure we don't lose any more titles on goal difference."