Kagawa first attracted attention from Europe's top clubs, including Manchester United, during his first season with Bundesliga outfit Borussia Dortmund. He netted eight goals in 18 appearances in an injury-hit campaign that ended with Die Borussen's first league title in nine years.
But it was his second season, Sir Alex told ManUtd.com, that cemented the United manager's belief that Kagawa had the necessary attributes to succeed at Old Trafford.
"There was a lot of interest in him after his first season because he started so well at Dortmund," the United boss said.
"But I never think you can tell after one season. I wanted to see how he’d do in his second campaign – it’s more of a test because your opponents tend to know more about you.