By ESPNSTAR.com staff
Kagawa joined the club from Borussia Dortmund over the summer and started life at Old Trafford smoothly with a goal on his home debut.
However, the 23-year-old has realised that he will need to improve physically in order to compete in the Premier League.
"After moving to the Premier League, I've realised that I need to limber up and strengthen my body trunk," the player was quoted as saying on AFP.
The Japan international is currently an injury doubt after sitting out their World Cup qualifying fixture against Iraq following a minor back strain - an injury that he played down as a minor one.
Kagawa told reporters before his long flight from Japan back to England: "It may be due to travel and fatigue. I can get by if I take good care of my body.
"I think I can come back soon. I have to consult my medical staff and do my best until the next match."
"I've learned a good lesson from this."
With a Champions League match against Galatasaray midweek and a tough fixture against Liverpool next week, Kagawa looks likely to be rested against Wigan on Saturday.
“Tough games are going to start and I will have to fight for a regular spot. I want to go out in perfect condition,” he added.