Kagawa had only scored twice for the Reds since his summer move from Borussia Dortmund but bettered that tally in one memorable afternoon with three clinical finishes past Mark Bunn.
With Wayne Rooney also getting in on the act late on with a stupendous strike, it was a great afternoon's work that sends the Reds 15 points clear of second-placed Manchester City.
"Midway through the second half, I didn’t think it would end up 4-0 as I think we were a bit ragged for that period," he told MUTV. "But, when we brought Shinji into central midfield, it made a difference to us and he ended up with a hat-trick.
"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. It’s been a great day for him."
The boss has already expressed an opinion that Kagawa will be even better next season after properly bedding in, which is an exciting prospect on Saturday's evidence.
"He got that injury around October-November and was out quite a while," continued Sir Alex. "It knocked him back a bit but he’s gradually coming back and I think you’ll see a really good player next year."
Rooney also earned praise for his efforts and focus can now switch towards the huge Champions League second leg against Real Madrid in midweek.