The Japanese international joined United from Borussia Dortmund in the summer of 2012, though has so far failed to hold down a place in the side under both Sir Alex Ferguson and more recently David Moyes.
The 24-year-old scored six times in the Premier League for the Red Devils last season, but has failed to find the back of the net this campaign.
"I still haven't scored any goals this season so I've felt very irritated," Kagawa is quoted as saying on Goal.com. "This is the first time in my career that I haven't been able to produce, so it's disappointing.
"I pressure myself to score in every game but some days it feels like I just can't win. I know I have to overcome that - if I can't it really shows that I haven't grown as a player. Because of that I have to really focus on the second half of the season and overcome that mental block."
Kagawa has made 19 appearances in the league for United so far this season.