Kagawa has made five Premier League starts this season but has revealed he struggled to gain fitness early in the campaign.
The Japanese claims he is approaching peak condition as he looks to bolster a United side currently ninth in the table.
"After playing in the Confederations Cup over the summer, I was a bit late joining the team. Under a new manager all the players were starting from scratch but I felt a little behind the others," he told the club's official website.
"It's hard to reach peak condition at the start of the season and the truth is I wasn't satisfied early on.
"My fitness is much better now and will improve even more in time. I'm not quite in optimum form but I'm feeling good."
Kagawa has played both centrally behind the striker and on the left for United in recent weeks, and the 24-year-old believes he can star in a number of roles for David Moyes.
"I played behind the main striker at Dortmund but I usually play on the left with Japan, and I also took this role at Cerezo Osaka," he added.
"I have some experience out wide but I think I should be flexible enough to play in a number of roles, so I'm a more attractive player to the team and can contribute even more.
"Every day I'm learning how to trouble my opponents on the left, how best to cause them problems with my movement, but I'd still like to be capable of taking up more positions."
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