The Japan international sealed a £17m move to Old Trafford from Borussia Dortmund in the summer but has seen his first-team opportunities limited by a knee injury picked up back in October.
The 23-year-old has scored two goals in 12 outings for Sir Alex Ferguson’s men but insists he has not been happy with his contribution since move to England.
“I’m not satisfied what I have done here at all.” Kagawa said. “It is a big and really famous club in the world, but I need to overcome lots of pressures and be mentally stronger.
“Unfortunately I got injured after a couple of months, but I have to be better and grow as a player and as a person in terms of contributing to team’s wins and succeeding here.
“We have a really strong squad with many world-class players, so it is lucky for me to have opportunity to train with them and play games with them, but I’m really feeling that I need to perform better.”
Kagawa made his return from injury in the 2-0 victory over West Bromwich on 29 December after more than two months on the sidelines with a knee problem.