The 23-year-old played the starring role as United thrashed Norwich City 4-0 to move 15 points clear of second-placed Manchester City, leaving top scorer Robin van Persie and Wayne Rooney in the shade with three classy finishes.
Kagawa, signed from Borussia Dortmund in June, has had his first season disrupted by a knee injury but showed signs against Norwich he could make a big impact at Old Trafford.
"It's brilliant for him. The lad is a good finisher and his second goal was so composed and an intelligent finish while his third was absolutely brilliantly-taken," Ferguson told Sky Sports television.
"It's been a great day for him.
"He got that injury around October, November and was out quite a while," added Ferguson who could be tempted to start Kagawa against Real Madrid in the Champions League last-16, second leg on Tuesday.
"It knocked him back a bit but he's gradually coming back and I think you'll see a really good player next year."
Kagawa, a regular for his country, is one of three Japanese players plying their trade in the Premier League along with Ryo Miyaichi, on loan to Wigan Athletic from Arsenal, and Southampton's Maya Yoshida.
United have a huge fan base in Japan and will tour there before the start of next season, playing Kagawa's first club Cerezo Osaka in a friendly.